Thanks very much for your input, everybody - none of it comes as a huge revelation, but it did me good to have my own suspicions supported by people who are or have been where I am right now.
The reason I haven't simply put a bullet in the relationship and walked away can be illustrated by my experience this morning. I went over to my BF's place last night (not to be crass, but I figure as long as he's taking his 'one last shot,' I'll go ahead and get me some while the gettin's good). This morning after he'd left for work and I was attempting to extract some toothpaste out of the beyond-empty tube that's been crumpled on his bathroom sink for two weeks, and threw the tube into the overflowing wastebasket, and then realized that he'd taken the expensive moisturizer I'd left at his place so I wouldn't have to bring it from home every time I stayed over, it struck me that the state of his home reflects the contents of his head - bits of unaddressed debris are everywhere. Laundry gets done, but never folded or put away. Items are purchased and put into use - with all the labels and tags still on them. Blinds are installed, but the pull-cords are never untangled. Dishes are rinsed, but never really washed. Moving boxes are unpacked...and then the empty box sits there for weeks.
These are all superficial things that don't *really* matter in the long run - and in an otherwise healthy 30-something person could maybe just be written off as foibles. But when you're just trying to brush your dang teeth and can't because a grown man (whom you just supported through six weeks of rehab) can't get it together enough to buy toothpaste, they seem more like sinister indicators of a deeply immature, unthinking, disconnected individual.
BUT...It also occurs to me that the half-done, half-assed state of BF's house could also be reflective of clinical depression - it may be that he can't bring himself to finish these tasks because he's just too depressed, and his drinking is a means of self-medicating. I realize it's not my job to diagnose him or be his home health aide (not only am I not interested in the positions, I'm not qualified to do either one) but the possibility of an underlying mental health issue is what introduces some doubt and prompted me to post the questions I did yesterday. If you're an immature douchebag who just can't be bothered, that's one thing - but if your brain chemistry has conspired against you, you may truly need an ally. Loyalty is one of my topmost values - so that's why I'm trying to figure out the true nature of the problem before I make my final move.
Annnnd, it's another novel-length post! So sorry, guys - I'm new here. I swear I'll wind down and be less long-winded eventually!