I agree with what daroogle said...when I was reading your post I kept thinking "good for you (less weight, out of school, etc. etc.), but what's that got to do with them?"

You think he's bad for her. You think you're better for her. You want it so much it hurts. But if you thought it was the right thing for anyone BUT you, Babe, you'd have done it already. She's got 8 years of history she's gotta get out of her system. She's doing it at her own pace. A friend might be the one to tell her B's not the one or that she can do better....a friend isn't the guy who mixes her up more by offering himself in B's place. 'Cause then....all this angst and confusion you've got now? You've given it all to her, all wrapped up with an even more guilt ridden bow. Not kind.

Two simple things to add.....first, if you and her are meant to be, it'll happen. Doesn't have to be now. Second, I don't know if it's the obsession with M...or that she really does dominate your personal social landscape as much as you describe....but it sounds to me like you've had her and her semi-functioning relationship front and center in your life for a damned long time now. Maybe, just MAYBE, the thing for you to do is spend some time with a woman more available. I don't doubt that she's the most "comfortable" person to be with...but it's not clear to me that you've given the rest of the gender a chance.

Just random thoughts....take 'em for what they're worth.