A similar thing just happened to me (guy getting back together with ex who he said is no good for him). I think sometimes people mistake feelings of nostalgia for love, and focus on the good times they had with a previous lover and how if only X would change, everything would be perfect, or if only X had happened, that person would have been "the one." This is not about you; this is about his inability to let go, to be in the kind of denial that allows him to sacrifice a loving relationship for a pipe dream of his "first love." Ultimately, you should pity him, and probably will. You don't want to be with the kind of person that could do something like that to you. Try to learn from his mistakes and don't paint the time you spent together with a rosy brush, ignoring the bad parts of your relationship, because a couple that breaks up and gets back together has already wasted the honeymoon period, and will soon fall into old patterns, remembering why it didn't work in the first place. The only situation I can imagine in which a broken-up couple can really make things work is if both parties have truly grown and matured to the point that the relationship is like new, and from what you've said, neither he nor his ex have changed all that much. I'm not in a position to give advice right now, so just try to gain joy from the things in your life that have nothing to do with him, that are completely your own.
Welcome to the board mimijiminy1569,
Here's your previous post from the other board:
http://messageboards.ivillage.com/iv-rlrelationsh/?msg=+29943.1
Wow.