The Agastya Nadi Jyotish revealed some of the sins I had made because of the bad karma I did in last life. I am a devotee and I chant GuruMantra everyday for an hour. Will it help me(people like me) escape the sufferings due to past life sins as it is said that taking Lord's name destroys sins? If one is ought to suffer because of bad Karma even if sins related to it are destroyed then what's the use of destroying sins by chanting the name of Lord? Is bad karma and sin synonymous? What Shastras say about this? I am using example of mine only to explain the question. I have no intention to gain personal advice. The situation like mine is generic and has been explained here just to provide the context.
If the sins are completely destroyed (by whatever means it might be) then how can you still suffer as a result of them?
When we commit sins we are guilty as long as we don't atone for them using valid methods. Once we have completed the atonement measures we are free from the guilt.
Manu Smriti 11.52. Thus in consequence of a remnant of (the guilt of former) crimes, are born idiots, dumb, blind, deaf, and deformed men, who are (all) despised by the virtuous.
11.53. Penances, therefore, must always be performed for the sake of purification, because those whose sins have not been expiated, are born (again) with disgraceful marks.
11.53 mentions if penances (prayaschittas) are not performed then the concerned person have to suffer from the consequences. But if done not so the case.
In your case, you are doing daily chanting of the Guru Mantra (given by your Guru), which is one of the valid methods of dissolving sins.
There are various methods which dissolve sins, one of which is recital of Mantras/Stotras etc.
11.227. By confession, by repentance, by austerity, and by reciting (the Veda) a sinner is freed from guilt, and in case no other course is possible, by liberality.
11.228. In proportion as a man who has done wrong, himself confesses it, even so far he is freed from guilt, as a snake from its slough
11.230. He who has committed a sin and has repented, is freed from that sin, but he is purified only by (the resolution of) ceasing (to sin and thinking) ’I will do so no more.’
According to my 1st Guruji, repeating the Guru given Mantra constantly in mind is the very best method of dissolving sins.
According to Kularnava Tantram, just doing the Japa of Guru's name is also a very good method of dissolving sins.
Now, in your case, the astrological calculations had revealed that you have done some bad Karmas in your previous life.
Let's denote that amount of sin by X. Now, you're regularly destroying some amount of X by doing regular Japa of Guru Mantra.
But, it's an extremely difficult task to ascertain exactly how many days of N number of Japa will eradicate that sin amount X completely. As long as even slight remnants of bad Karma is present in our account we always have to pay for it.
So, the real question here is not "if by devotion I have eradicated all my past life sins, will I still have to suffer from it?" but it is "How efficient is my method in eradicating a particular amount X of sin?"
The answer to the first question is clearly a NO as shown above but the second question is not that trivial to answer. Because the answer depends on lot of factors like 1. amount of devotion you have 2. efficacy of the Mantra you're doing the Japa of 3. the number of times you are doing the Japa 4. efficacy of the initiation (Diksha).
The effect of the karma will wear away through devotion to God. Suffering of the devotee will be greatly lessened.
Nanda: "But how can we obtain God's grace? Has He really the power to bestow grace?"
Sri Ramakrishna (smiling): "I see. You think as the intellectuals do; one reaps the results of one's actions. Give up these ideas. The effect of Karma wears away if one takes refuge in God. I prayed to the Divine Mother with flowers in my hand: 'Here, Mother, take Thy sin; here take Thy virtue. I don't want either of these; give me only real bhakti. Here, Mother, take Thy good; here take Thy bad. I don't want any of Thy good or bad; give me only real bhakti. Here, Mother, take Thy dharma; here, take Thy adharma. I don't want any of Thy dharma and adharma; give me only real bhakti. Here, Mother, take Thy knowledge; here take Thy ignorance. I don't want any of Thy knowledge or ignorance; give me only real bhakti. Here, Mother, take Thy purity; here take Thy impurity. Give me only real bhakti.'"
The Gospel of Sri Ramakrishna, chapter 43, Visit to Nanda Bose's House, July 28, 1885, p 817