There's three divisions of Karmas:
Kriyaamaana (free-willed karma), Praarabdha (karma that has been propelled forward) and Sanchita (the total heap of Karma)
Sanchita karma is a total collection of all Karmas that you've done, and the Phala of that Karma is waiting to fructify.
A part of that Sanchita is taken to be experienced in the form of Praarabdha, this is what decides in what Loka you're born in, in whose family, in what time, in what land, with what health, with what type of relationship to your parents, what kind of relationships you have with friends, how intelligent you are, how strong your memory is, how much money you are born into, how much money you get, what kind of body you get (deva/human/ant/bacteria) etc.
Now, within the constraints of the Praarabdha, you have the ability of committing new karma. An ant can never produce the karma of helping thousands of other ants. A human can create a karma of helping billions of others. Clearly, the Praarabdha is a limiting/constraining factor as it decides the body through which you act in the world. That mental, physical and soul (Ahamkaara) form is what experiences the karma, and that same form is the one that commits new Karma.
We know that karma committed as an accident doesn't count in this. For example, you stepping on an ant without realizing, you're not responsible for that sin, it doesn't get added to your Sanchita via the Kriyaamaana. Why is this? It's because your Ahamkaara hasn't identified itself with the Karma. There's no cognition of 'I did this'/'I am doing this'. This is the same reason why enlightened people cannot commit new Karma, good or bad, they're simply going through Praarabdha in a spontaneous way without self effort of the Kriyaamaana. Their Ahamkaara is constantly identifying itself with (Satya Jnaana Ananta) Brahma(n), not with the actions happening in their Ahamkaara (via what it identifies with), or Mind(thoughts/emotions/plans) or body.
Now, an interesting question arises. You didn't get the Karma for the ant dying. But, the ant still did die, right? So, what caused the Ant to die? You can think of it like a natural disaster for humans. As far as we can tell, no one really caused an earthquake, but lots of people died in the earthquake. But, the ant can only die if it was in his Praarabdha to die in that manner, correct?
So, here we can see that you're being used by Ishvara to give Karma Phala to the ant. You're not being held responsible for the good or bad that you did unknowingly. But, good and bad are still occurring to others via you. Of course, Ishvara is not controlling you (to a large extent), it is still based on your own patterns and habits and tendencies and nature of your body that something like this would occur.
So, to come to the main topic:
Are our actions our own or does Ishwara use humans as mere instruments to carry out certain actions?
The answer is that both are true at once.
But, a question that you may ask is, how do you know that your Karma is really yours? What if determinism is true and it just feels like you're doing something, when you actually have 0 control.
Well, then Ishvara would be really cruel, would he not? What even would be the point of Karma? As just some sort of tracking mechanism? This is simply not true, if you're already accepting Hinduism as true, then it cannot be true.
Because, multiple scriptures tell us to do Purushaartha (put in effort to achieve something) and not just rely on Daiva (destiny/fate). If free will wasn't there, then would there even be a point in that?
For example, here's a Gitaa (excerpt) from the Mahabharata.
पितामह महाप्राज्ञ सर्वसास्त्रविशारद।
दैवे पुरुषकारे च किंस्विच्छ्रेष्ठतरं भवेत्॥ 13-9-1
वसिष्ठस्य च संवादं ब्रह्मणश्च युधिष्ठिर॥ 13-9-2
दैवमानुषयोः किंस्वित्कर्मणोः श्रेष्ठमित्युत।
पुरा वसिष्ठो भगवान्पितामहमपृच्छत॥ 13-9-3
ततः पद्मोद्भवो राजन्देवदेवः पितामहः।
उवाच मधुरं वाक्यमर्थवद्धेतुभूषितम्॥ 13-9-4
पर्णान्नालाः प्रसूयन्ते नालात्स्कन्धः प्रवर्तते॥ 13-9-5
स्कन्धात्प्रवर्तते पुष्पं पुष्पात्संवर्धते फलम्।
फलान्निर्वर्तते बीजं बीजात्स्यात्सम्भवः पुनः'॥ 13-9-6
नाबीजं जायते किञ्चिन्न बीजेने विना फलम्।
बीजाद्बीजं प्रभवति नाबीजं विद्यते फलम्॥ 13-9-7
यादृशं वपते बीजं क्षेत्रमासाद्य वापकः।
सुकृते दुष्कृते वाऽपि तादृशं लभते फलम्॥ 13-9-8
यथा बीजं विना क्षेत्रमुप्तं भवति निष्फलम्।
तथा पुरुषकारेण विना दैवं न सिध्यति॥ 13-9-9
क्षेत्रं पुरुषकारस्तु दैवं बीजमुदाहृतम्।
क्षेत्रबीजसमायोगात्ततः सस्यं समृद्ध्यते॥ 13-9-10 Source
Translation: Yudhisthira said: Grandfather, of great wisdom, and an expert in all Shastras, which is superior between Daiva (destiny) and Purushaartha (free-willed efforts of man)
Bhismha said: There's a history of great antiquity that exemplifies the answer to your question. Yudhisthira, this is about Vashishta's conversation with Brahmaa. Long ago, Vashishtha asked Brahmaa, which Karma is greater between Maanushya (karma derived from man) and Daiva (karma derived from Devas)? Then, king Yudhisthira, the grandfather of the whole world, who was born from a lotus and who is the Deva of the Devas, said with a sweet voice something that contained great meaning and was of great reason. Brahmaa said: From a seed, a sprout arises, from a sprout, leaves arise, from leaves, a pseudo-trunk (hollow tube) arises, from a pseudo-trunk, a trunk arises, from a trunk, branches arise and from branches, flowers arise. From the flowers, fruits arise and from fruits, seeds arise. The seed is said to never be fruitless (i.e the seed contains the fruit). Nothing arises without the seed, not even the fruits. The seed creates other seeds and the seed is understood to create fruits. The type of seed a farmer plants in the field leads him to the appropriate fruit. In the same way, good and bad deeds certainly lead to the appropriate (good or bad) fruits. Just as a seed cannot produce fruits without the field, the actions of man cannot produce fruits without Daiva (aka Praarabdha). The field is said to be the actions of man, and the seed is said to be the Daiva. The crop is only produced after a correct union between the field and the seed.
The thing to take out of this whole thing is, that it clearly suggests that there is free will involved somewhere. If it was just talking about what happens when people do karma in favorable situations, then it didn't need to be in the Mahabharata, any random guy can observe it, the involvement of Brahmaa is not needed. So, we have to take the meaning as the contention between what has been pre decided, or rather not under your control, and in the control of the Devas (Praarabdha) and what the opposite of it is (free willed Kriyaamaan Karma).
But. maybe this isn't convincing enough. In which case, what is the point of separation between the Praarabdha and the Kriyaamaana Karma? If everything was predecided, then all Kriyaamaana would be Praarabdha itself. The actions happening to you (and appearing to you as you doing it yourself) would be in the same category as people doing things to you (which is under Praarabdha) and the body being given to you (also based on Praarabdha). But since the categories are different, we can say with confidence that there's a portion of Karma that is based on free will.
Now of course, it differs based on many factors. While in sleep, you're doing 0 Kriyaamaana Karma, yet actions are happening through your body. An ant, the vast vast majority of it's Karma is not Kriyaamaana, simply because it doesn't have the capacity to make free willed decisions. Part of its Praarabdha is to just execute patterns based on pheromones, day/night cycles, orders of the queen and so on.That appears as Karma done by it, and it is, but it's not being counted as Kriyaamaana and not being added to the Sanchita.
That's where the idea of Karma Yoni and Bhoga Yoni comes in. There's certain births, where you can commit Kriyaamaana Karma (such as being a human), and there's certain births where you just reap the Karma Phala without being given an option to commit new karma (the body of a bacteria or plant).
This probably still isn't enough to dispel the hesitation either, and there's really no evidence that can convince you that your life isn't pre-decided. But the better question is:
Why are you even entertaining the possibility that life is pre-decided when it CLEARLY feels like you're the one making the decisions, moment to moment?
Does it not seem a bit stupid? This is the same sort of argumentation materialists use to say mind and consciousness comes from matter, when it clearly feels like mind is separate, and not to mention, there's no evidence for such a thing. Just as there's no evidence for determinism.
If it's the 3 gunas (qualities of Ishwara's prakriti) that compels us to act in good and evil ways, then why do the jivas have to reap the results?
The 3 Gunas do compel you to do actions however, they do not decide what actions you do.
For example: the father of a child may compel him to study, but the child may play instead. This is the same sort of relationship between Gunas and you.
Just as the father pushes the child to do study (Sattva), the brother pushes the child to play (rajas) and the mother pushes the child to sleep to wake up early tomorrow (tamas)... the three Gunas act. In the same way this happens, the Trigunas change your mental state to when you may want to sleep 10 hours or sit on a sofa for 5 hours and watch a movie(tamas), you may want to go and play and exercise and beat up other people (rajas) and where you may want to just pray, do some work that needs to be done and meditate (sattva). In no situation are you forced to do something. You always have the option to not sleep even if tamas is strong. Only in extreme situations of tamas (such as falling unconscious out of exhaustion), do you not have a choice, but in that case, that karma (and subsequent Phala) is not attributed to you anyways, as you do not identify with it.
TriGunas DO NOT definitvely control your choices, although they certainly do influence them. They're more like structures for you to act inside, similar to the Praarabdha. Trigunas are a part of Praarabdha actually, as well.
No decision exists in a vacuum, there's always factors that influence the decision, the majority of the factors are decided by Praarabdha or Karmas you chose to do previously (in this life) that caused you to get into the current mental or environmental situation.
Hopefully that answers the question.