The problem is that chaotic systems do not exist barring QM. Every single movement of every single atom in the universe is determined by what happened directly before that movement. I mean, I see what you're saying, but I don't think you understand how psychological determinism works. All the thinking about other "itches" as you say is a result of what memories and behaviors the original "itch" brings up through neural pathways. Undoubtedly it is extremely complex, but what you seem to be arguing is that complexity means psychological determinism cannot be true.
Perhaps there is something I'm missing about the argument for psychological determinism, in which case I'm all ears... But to me it seems this: I have desires/needs, whichever proves strongest is the one I inevitably follow, and there's my "decision". And I can understand that: our neural pathways are electro-chemical rather than magical, and they follow the usual laws of physical causality - put in X input to get Y output (to a certain extent, at least - those electro-chemical processes still have a degree of variance.) But while the individual actions may be determined, I think the dynamics between them cannot: my argument is simply that there are so many competing desires in a human (not just the basic physical sensations like hunger, but also guilt, greed, etc.) that the end result is a chaotic mess resulting from the unending competition of those desires. You may make a "decision" one minute, and arrive at a completely different "decision" the next, which implies that each "decision" is actually a heuristic evaluation dependent on how long you had to think about the problem. The wider I consider my possibilities, the greater my scope for making the "best" decision - but it is still
my choice to look beyond the obvious or not.
But, then again, I don't have the benefit of studying the topic in depth, so if I've missed the crux please enlighten me.