Free will cannot exist.

Is a mental activity determined by anything or not? If it's undetermined by anything then it's random and so by definition you're not in control. If it's determined by something then it's either determined by something 'deeper' in your mind or external to you. If it's external to you then you're not in control again. If it's deeper in your mind then all you do is push the problem back and you can simply ask the question again 'is this determined or not?'.

If you take the stance that the chain of determinism terminates at something like the soul, unfortunately that hasn't solved the problem, because the problem doesn't lie in explaining the mechanism by which decisions are made, that could be a complete magical mystery and that would be fine, but rather the problem comes from the chain of determinism that must either terminate at something outside of your control or random. Neither of these options allow for free will.