Two things define you as an individual: your mind and your body. Put another way, “nature and nurture.” You were born with a set of potentials (like intelligence or musical talent), just like you were born with certain physical characteristics (like gender or eye color). These potentials influence or predetermine the outcome of your life to some degree.
Your personality is also molded by the experiences you have in life—especially in your early life—particularly in your relationships with other people. In your early years, those potentials were formed by your experience into what you think of as “you.” These experiences can have a “predetermining” effect on your present reality, too.
Ultimately, though, you determine the events and relationships in your life, apart from any outside influences on your choices. Nothing is truly predetermined; anyone can radically change the course of their life at any moment. Even if you can’t change the characteristics you were born with (like intelligence or shoe size), you can change the ones that your environment shaped (like political views or health).
What you do in life is create a present reality for yourself by choosing from the possibilities “out there” in your daily experience, based on the values you hold “in here”—in your mindset. Your beliefs are one kind of value you’ll find in there. Beliefs are so powerful that, if someone really believes their life is predetermined, this belief will be reinforced by their life experience.
But so will a belief in free will. Or anything else!
If you learn to overcome your beliefs and other false impressions, you can see what’s really happening here: your reality is being created every moment by your mindset (which includes your beliefs, opinions, assumptions, and so on). Then questions like Free will or destiny? and Nature or nurture? won’t be an issue. You will be in control of your own life, and you will see how your experiences contribute to your own decision-making.