First of all, thanks for the input (and to PaperDolls for the compliment!)...it's been validating and very helpful - in a very depressing, hope-killing sort of way, but nevertheless, it's much appreciated.
I granted my BF the 'one last shot' he begged me for, and I intend to see that through to its conclusion. In the meantime, I'll likely attempt to initiate a conversation about the possibility of clinical depression. Despite all the misery of the last few weeks, he and I *do* have what I consider better-than-average communication most of the time, so it's not like I'd be nervous to broach the subject. It will probably amount to a big zero, and may even hasten the end of the relationship, but it certainly can't hurt at this point. If he makes an appointment with a therapist, that's fabulous - if not, we're still sitting here with the clock ticking and the inevitable end bearing down.
And all of it just in time for my birthday tomorrow - wooo-hah! *facepalm*