OK, what do I mean by free will?
I would define it as the ability to perform mental functions (e.g. agency, creativity, decision making) in a manner that is not determined by things that have already happened, i.e. the past
I believe this is impossible, hence I don’t believe in free will
Well, if you define the term in such a way as to agree with your objections, it's easy to prove your point. However, 'free will' isn't about being able to act without outside influences. "Free Will" is about being able to choose between options actually available to us. Under your definition, 'free will' might well include the ability to decide to be flying around, 10,000 feet up, under one's own flapping arms without any mechanical aids.
In reality, 'free will' is about being able to judge between influences and desires in order to pick among the logical and possible options in front of us.
In fact, if there were no free will, no robber or kidnapper would have to hold a banker's family hostage in order to force him to rob his own bank. A kidnapper wouldn't have to steal away a family member in order to coerce someone to pay money to get that family member back.
Even so, the choice is there; does the banker or relative have the choice between rob...and not rob, pay the ransom, or not pay? Of course s/he does. If s/he didn't, there would be no need to, literally, hold a gun to the heads of the people being threatened if s/he makes the wrong (not paying) choice.
...and still the FBI and other law enforcement people tell you not to pay, but to leave things in their hands. What a choice...but it IS a choice.
Free will isn't about having every and any choice. It's about being able to choose at all, knowing that one's choices will have consequences. You don't pay the ransom? You will be mourning a family member. But the choice is yours...nobody is going to tie a puppet string to your arm and use you to pay it, will you, nil you.
No sir, free will is the ability to choose between available options...with the consequences of those choices...and there are always, and I mean always, available options, right up until unconsciousness or death. Perhaps extremely constrained, to be certain, but they are there.