Unfortunately for you, in this not quite parallel universe, conventional logic does not apply. For a woman, it makes perfect sense to dump a nice guy for an alcoholic ex-flame. Oh, sometimes she'll play along and admit that it may be poor judgement, but then she'll claim that she can't help herself. Or, maybe, she'll be the one who changes him. Or even -- who knows? -- it's possible that he'll quit drinking, find a cure for cancer, feed the Third World and become the perfect boyfriend. Who are you to judge, buddy? Why I bet you never cared for her anyway. If you did, you would be more worried about her happiness than about criticizing a nice guy who's got enough problems of his own to deal with thank you very much.
Sorry, I got carried away. It's a seductive little universe, theirs.
So, what can you do? For a start, develop your own personal and inter-personal problems. Not necessarily alcoholism; there are a lot of options out there: start knocking over convenience stores, maybe try heroin (hell, you might like it), or whatever. Be creative. Will you win her back? No. But you might get some other guy's beautiful, charming and intelligent woman.
Since we're on the topic, you might as well know that there are other over-laid universes as well, although the others are increasingly further removed from your own. Perhaps the most obvious one is the world of cats. Yet another one is -- well, I don't want to scare you. Let's just say it's a universe where Windows 95 is the most amazing piece of -- no, really, I guess you've had a bad enough day as it is.
You owe the Oracle a dozen long-stemmed roses. 'Cuz hey, you've already lost your chick, but I really need those roses for you-know-who. And toss in some chocolates. No, they're for me.