Thanks to all who responded! And I agree with everything each of you has said.
Eventually, I'll sew this thing together and either be happy with it or haunted by a "mistake". I think I'll be happy enough, knowing with a quilt like this, there is no perfection, that the fabrics I've used -- that purple and that red and white striped particularly -- are always going to make arrangement less than perfect.
I've already moved the blocks in which the purple makes a 3 sided box. The idea of setting a time limit is good; I think I'll do that. Actually, I've done that before but without thinking it out, in that I've tinkered with something for 2 or 3 days and then suddenly decided: that's it, no more, I don't care (pick one! ha!). What really happens is that I finally get so bored with it and my issue with it, that I make a final decision.
I can't tell you, though, how encouraging it is to know that I am not alone in this kind of obsession, whether it's based in perfectionism or good design or OCD. It's nice to know there is understanding and sympathy....and most important, ideas that help! Thank you all!