Doesn't our whole life is influenced by our previous karmas?
Our life is influenced by prarabdha karma (karma of previous life) but they do not completely influence our life. Prarabdha can only contribute or counteract in our karma. But it cannot stop us from doing karma. Suppose if a person is going for an important work in car and suddenly his car stop due to mechanical problem. This is because of his prarabdha karma. But now it depends on his free will that whether he give up and loses his hope or try to reach his destination through any other means or on foot. Prarabdha is also an account of our karmas in past life. Existence of prarabdha is itself the proof of our free will. I am quoting a passage from Anushasan Parva Section 6 to prove my point.
Brahma said, 'Nothing comes into existence without seed. Without seed, fruits do not grow. From seeds spring other seeds. Hence are fruits known to be generated from seeds. Good or bad as the seed is that the husbandman soweth in his field, good or bad are the fruits that he reaps. As, unsown with seed, the soil, though tilled, becomes fruitless, so, without individual Exertion, Destiny is of no avail. One's own acts are like the soil, and Destiny (or the sum of one's acts in previous births) is compared to the seed. From the union of the soil and the seed doth the harvest grow. It is observed every day in the world that the doer reaps the fruit of his good and evil deeds; that happiness results from good deeds, and pain from evil ones; that acts, when done, always fructify; and that, if not done, no fruit arises. A man of (good) acts acquires merits with good fortune, while an idler falls away from his estate, and reaps evil like the infusion of alkaline matter injected into a wound. By devoted application, one acquires beauty, fortune, and riches of various kinds. Everything
can be secured by Exertion: but nothing can be gained through Destiny alone, by a man that is wanting in personal Exertion. Even so does one attain to heaven, and all the objects of enjoyment, as also the fulfilment of one's heart's desires by well-directed individual Exertion. Al! the luminous bodies in the firmament, all the deities, the Nagas, and the Rakshasas, as also the Sun and the Moon and the Winds, have attained to their high status by evolution from man's status, through dint of their own action. Riches, friends, prosperity descending from generation to generation, as also the graces of life, are difficult of attainment by those that are wanting in Exertion. The Brahmana attains to prosperity by holy living, the Kshatriya by prowess, the Vaisya by manly exertion, and the Sudra by service. Riches and other objects of enjoyment do not follow the stingy, nor the impotent, nor the idler. Nor are these ever attained by the man that is not active or manly or devoted to the exercise of religious austerities. Even he, the adorable Vishnu, who created the three worlds with the Daityas and all the gods, even He is engaged in austere penances in the bosom of the deep. If one's Karma bore no fruit, then all actions would become fruitless, and relying on Destiny men would become idlers. He who, without pursuing the human modes of action, follows Destiny only, acts in vain, like unto the woman that has an impotent husband. In this world the apprehension that accrues from performance of good or evil actions is not so great if Destiny be unfavourable as one's apprehension of the same in the other world if Exertion be wanting while here. Man's powers, if properly exerted, only follow his Destiny, but Destiny alone is incapable of conferring any good where Exertion is wanting. When it is seen that even in the celestial regions, the position of the deities themselves is unstable, how would the deities maintain their own position or that of others without proper Karma? The deities do not always approve of the good deeds of others in this world, for, apprehending their own overthrow, they try to thwart the acts of others. There is a constant rivalry between the deities and the Rishis, and if they all have to go through their Karma, still it can never be averted that there is no such thing as Destiny, for it is the latter that initiates all Karma. How does Karma originate, if Destiny form the prime spring of human action? (The answer is) that by this means, an accretion of many virtues is made even in the celestial regions. One's own self is one's friend and one's enemy too, as also the witness of one's good and evil deeds. Good and evil manifest themselves through Karma. Good and evil acts do not give adequate results. Righteousness is the refuge of the gods, and by righteousness is everything attained. Destiny thwarts not the man that has attained to virtue and righteousness.
Further it is said that both destiny (prarabdha) and exertion of person affect his life.
Destiny does not help the man that is steeped in spiritual ignorance and avarice. Even as a fire of small proportions, when fanned by the wind, becomes of mighty power, so does Destiny, when joined with individual Exertion, increase greatly (in potentiality). As with the diminution of oil in the lamp its light is extinguished so does the influence of Destiny is lost if one's acts stop. Having obtained vast wealth, and women and all the enjoyments of this world, the man, without action is unable to enjoy them long, but the high-souled man, who is even diligent, is able to find riches buried deep in the Earth and watched over by the fates. The good man who is prodigal (in religious charities and sacrifices) is sought by the gods for his good conduct, the celestial world being better than the world of men, but the house of the miser though abounding in wealth is looked upon by the gods as the house of dead. The man that does not exert himself is never contented in this world nor can Destiny alter the course of a man that has gone wrong. So there is no authority inherent in Destiny. As the pupil follows one's own individual perception, so the Destiny follows Exertion. The affairs in which one's own Exertion is put forth, there only Destiny shows its hand. O best of Munis, I have thus described all the merits of individual Exertion, after having always known them in their true significance with the aid of my yogic insight. By the influence of Destiny, and by putting forth individual Exertion, do men attain to heaven. The combined aid of Destiny and Exertion, becomes efficacious.'"
It is said in Vana Parva Section 32 that one should not rely on his prarabdha;
All the creatures in the world would have been exterminated, if there were no action. If also acts bore no fruits, creatures would never have multiplied. It is even seen that creatures sometimes perform acts that have no fruits, for without acts the course of life itself would be impossible. Those persons in the world who
believe in destiny, and those again who believe in chance, are both the worst among men. Those only that believe in the efficacy of acts are laudable. He that lieth at ease, without activity, believing in destiny alone, is soon destroyed like an unburnt earthen pot in water.