Part of the problem is deciding what it means to have 'free will'. Another part seems to be thinking that you and your personality are different than the processes happening in the brain and body.
So, when 'I' make a decision, what is actually going on? I am processing the information I have, including my desires and values, and determining what it is I want to do. If 'I' am the one that primarily makes this determination, then 'I' have free will.
But, that is exactly what happens: the relevant processing to determine what I will do happens in *my* brain. It isn't determined by an 'outside force', but the relevant processes happen *in me*. Furthermore, those processes are not 'pre-determined' in the sense that an outside observer previous to the decision would be able to determine what the decision will be (at least, not always). The level of description that would be required to 'calculate' the decision based on *all* the inputs would be immense; even leaving out a few of the inputs would potentially lead to a different decision.
Hence, I am the one making the decision and that decision is not pre-detemined in any reasonable sense. So there is free will.