I've been there. Truth be told, sometimes I still am. My thoughts are that you're thinking of giving up a relationship that you know is good for what is effectively a fantasy. The possible relationship with your ex is run through your own "what if" filter for lack of any better term. I don't know the circumstances you guys split up under, but there were reasons obviously. Maybe they were because of behavior that's now different; dunno. Remember she may be a different person now too. That's what I mean about the fantasy. She may not be who you're envisioning, with the all the "canonization" that can go on in your head when you think back on her. You may be in love with a memory. Not sure how else to put it that makes any sense. On the other hand, you get one shot at life, and if this weighs on you that much, I would never tell you not to pursue something that can make you happy. I would just caution you to think about what you really miss: the girl from years ago, or an idea. Hope that helps a little.