Definition of Free Will (Assumption): We assume having free will means having the ability to pick one or more choice(s) (but not all) out of definite or indefinite choices; that one or more choice(s) could be any of the various options.
The Paradox: The only way to prove that you have the ability to pick a (any) choice is by picking that (one) choice, and since you can only pick one or more (but not all) of the various choices, you can't prove that you had the ability to pick anything other than those particular choice(s) already made.
Example: I give you a cup of fatal poison and ask you whether you have the ability to drink the cup. You can reason that you do have the ability but you won't drink it because of common sense. I argue that you don't have the ability to drink it because common sense prevents you from drinking it no matter how hard you try. You can't prove that you have the ability to drink it because you won't do it, so you don't have the ability to drink that cup. Consequently, you don't posses the free will to do so either. This example can be expanded to everything (minus choices already made).
Conclusion: Since you can't make all the possible choices there is to make, you can't prove that you had the ability to choose any of those choices (other than the ones already made), therefore you had no control over the choices already made. Consequently, you don't posses any free will. By proving free will exists (making all possible choices at the same time), you also prove that it doesn't exist because if all choices can be made simultaneously, then the definition of free will would be invalid.

Does that make any sense to anyone here?