And the part of all this that keeps you sane is that, although everything that happens has been determinate for eternity, no-one is or ever will be capable of knowing what will happen in the future (although that does not mean that educated and approximate guesses cannot be made)
James, your basic premise seems to be that the future is determinate. The notion of a clockwork universe was discarded with the advent of quantum mechanics. We can make statistical predictions, but the outcome of individual quantum events cannot be determined no matter how much you know about the initial conditions.
Chaos theory tells us that quantum events in the early universe could have a tremendous effect on its subsequent configuration and therefore all of the events which led you to make a particular choice.
According to Everett's "many worlds" interpretation of QM, the universe splits every time a choice is made and all possible futures exist.
If a mind uses quantum processes to make ANY decision, then the future is not predetermined by the past. For instance, if you are equally inclined to bet your kids' college fund on black or red at a casino, your final decision may depend on a random neutron firing, and whether you win or lose is determined by the total momentum of a collection of atoms which are subject to Heisenberg's uncertainty principle.
I don't know of anyone (other than those who believe in an omniscient god) who would categorically state that the future is determinate. If it is, you have only the illusion of free will since it is impossible to will yourself to make any choice other than the predetermined one.