There are three types of Karma. Sanchit Karma is the total accumulated stock of one’s past karmas. Prarbdha karma—what we call as Destiny---is the part of Sanchit karma which is passed on to us in this birth for suffering or enjoyment. And there is Agami karma which is performed in this birth, out of free will.
There is the example of the Bow & arrow. The accumulated stock of arrows in the archer’s case slung over his shoulder is the Sanchit karma.The arrow that has just left the bow is Prarbdha Karma and the arrow in his hand is the Agami karma. The arrow that has left cannot come back. Hence Prarbdha karma. For the arrow in hand he can still decide whether to use or not and on whom to use. Thus he has Free will on the arrow in hand. Hence Agami karma.
"But, how do we know what is covered in Prarbdha or destiny? Is death or major disease or wealth part of destiny? If death is predecided then what is role of Medical science?” The spiritual Guru replied “There is role of both karma & free will in all aspects of life including in trying to save someone from the jaws of death. At the time of action, one should not think of one’s destiny, because no one can predict it.”
“If results are not achieved in spite of all efforts then it can be attributed to Prarabdha. The theory of destiny helps to accept results without frustration, whenever the results are not achieved inspite of all efforts(Agami karma). At the same time it does not make a successful person egoistic.”
The example of Rama deciding to go after the golden deer is given to explain how the course of destiny that was to happen influenced his free will.
“Does this mean that karma fails in front of Destiny?” The Swamiji replied, “No,arma never fails. Although it appears to have failed in achieving the immediate worldly results, all karmas in any case getting added to the Sanchit Karma, out of which future results can be improved.”
Hence do your best always and leave the rest to God.