Good for you - getting the degree and not being intimidated by the "traditional student" thinking that says your too old. I did mine like you, in my late 30s and early forties, at two classes at a time, and sometimes it was just insane. On the coffee thing, you're right - it's probably not having the same effect. I know, "coffee and its effects", right? WTF some might say, but when you've strapped yourself into the commitment and mentality required to finish something like this, these little things can be BFDs sometimes. Back to the coffee - you in all probability have reached (and have gone past) a point of diminishing return. When I would do this (and still do occasionally) I'm always amazed at how rested I feel about two weeks after just saying no to caffeine. The kicker is, is that if/when you have your next cup, it will be everything you had dreamed it would be and more. Seriously I would grab quads at S'bux and when I would go back to that after a coffee hiatus, holy shit.

Anyways, hang in there and best of luck!