I start at the other end, and agree with luanqibazao. I consider and reflect on my experience, and find I have free will. That there are things I did, when I could have not done them. How various sciences and philosophies want to make sense of that is up to them, if they can..
I think it is very hard to actually engage with the idea that everything one does - and I include conscious thought here - is predetermined. Indeed, surely, definitionally, it's impossible? Certainly, it is not 'thought' as experienced as a live, active, selecting and considering and pondering and deciding thing, but rather 'thought' as a series of predetermined notions that make themselves known to, well, themselves - or at best, to a conscious self that is something like a dog watching a card trick: it's just a bunch of suff happening, but at least it in some way registers that it is happening.
This is what I meant by saying that, either I am right to believe I have free will, which is great, or the whole shebang is predetermined and belief is impossible, as it suggests volition and active engagement. One does not believe in determinism, rather, the state of 'belief in determinism' occurs and makes itself known.
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Of course, it's probably quite a bit more complicated than that.
You know sometimes, between the dames and the horses, I don't even know why I put my hat on.