We always do what we want. However, circumstances can narrow down our choices...
I'm sure you have seen the argument against God's omnipotence which goes something like this. If God is omnipotent then is God able to make a rock so heavy that He cannot lift it?
I don't know if people use this argument in a tongue in cheek way or not, but I suppose some people would be serious in their use of it.
The argument is however based on words and a misunderstanding of God's omnipotence.
In the language it is implied that God's omnipotence means that He can do whatever is possible to do, not those things that are by definition impossibilities.
So the inventor of the argument first of all makes that mistake in the premise and then goes on to state something that is impossible to do and then claim that God is not omnipotent because God cannot do it.
The same sort of thing happens in arguments denying the omniscience of God and in those like you argument about determinism and basing it on our wants.
Free will for a start is being able to do whatever we want to do.
The implication of course in that is that we are able to do whatever we want to do that is possible to do.
So those who have made up the argument for determinism, just like in the omnipotence argument, leave out the "that is possible" part and then go on to say that we cannot do what God does not foreknow, or cannot do what we do not want to do, or cannot do something other than what will happen, therefore free will does not exist.
But of course, as you probably realise, it is, by definition, impossible to do what we do not want to do, because whatever we do must be what we want to do.
It is impossible to do something other than what God knows we will do if God is omniscient.
It is impossible to do something other than what will happen.