My advice to all ya'll out there: do or do not have sex. Whatever you want to do, think about it deeply, then make a decision and then abide by it. I rarely hear people say "you know, I sat down and thought about it for a long time, considered my options, made a decision, and now I totally regret it."
No, the regrets usually seem to be a result of hasty or ill-informed decisions-- maybe not always, but almost always.
As to the OP: you made a decision and it sounds like you gave it some thought. Good for you: you probably did the right thing.
Plus, you gotta figure that if something should happen to change your circumstances, the opportunity may still be around. (After all, it's not like she is going to stop being attracted to you because you turned her down once, particularly if you gave her the old 'I find you super attractive, but right now the timing sucks...' routine. And if she does stop finding you attractive, odds are, her reasons for wanting to get into your pants weren't really about you in the first place.)