It's an age old question and despite answers from great sages, I've been trying to grapple with this question myself. First, I realise it's important to point out that I believe in reincarnation; that we have many lifetimes, the purpose of which is to bring us to awareness of our own divinity. By this I mean that we are made in the image and likeness of God, that we are divine, not that 'he' is human, and prone to the petty jealousies and anger of humans!
Within this context, I wonder if, as souls, we decide on certain key events that will occur during our lifetime, when we incarnate. The soul needs to preplan these because the personality is living unconsciously, with no communication between the soul and personality, with no flow of intuition and no awareness that our thoughts create our reality.
These events form our destiny, they are the fundamental experiences our souls wish to have in order to understand fully what it is to be human, and consequently, what divinity is too. However, we can reach these moments by going down different paths, some of them are easier and some of them are harder, the choice is ours: that is our free will.
As we become increasingly conscious of our journey while in human form the balance between destiny and free will evolves. As lifetimes progress, and we become increasingly conscious, we develop greater free will as the soul no longer has to preplan the lifetime in the same degree of detail, because the personality begins to create their lives more consciously, no longer being swept along by the tide of life. The soul and the personality come into communication, and the personality creates with awareness and increasing care, as it realises that what it does affects not just those around them, but all others on this journey too.
I've no idea if this idea represents reality or not. It's just a wondering...