I know in my case, as much as I really really wanted to get back together with my ex-boyfriend, i had to accept that the issues that were a problem were still there. He really hadn't moved forward since we'd broken up. The only self-realization he'd had was that he had pushed me away and he regretted it. But i had no guarantee he wouldn't do it again.... I just couldn't rationally see the evidence that he was going to face some of the shit in his life and deal with it, and that's what i needed him to do before I could open that door back up again. So i went with my head and not my heart. I haven't regretted that decision, I mean I did find someone whom I am now married to who is able to be the person that I need. I don't really believe in "The one' or any such nonsense....I mean if me and my ex had met 10 years later than we did, we probably would have lasted for years, if not a lifetime. But I've moved forward and found the happiness I want in someone else. I dunno, that's my experience.
I actually was thinking the other day (i guess my husbands hospital visit got my mind going down morbid paths, lol) like what would happen if my husband died or something and I found myself single again. I decided I would 100% for certain look up my ex and see how he was. Even though the "love door" is shut to that guy, I'd be open to opening it again if I was single. (that's the rational mind decision,.....i'm not a cheater, so all you jealousy-types don't go judging)