Compassionate Dumper,
Second Best Syndrome (SBS) is to long-term monogamy what HPV is to sexually transmitted diseases. In the majority of instances it's non-threatening, and almost everyone has it at some time or another.
SBS can even be healthy. It's a temporary mental escape, an opportunity to vent the boredom and frustration that inevitably arise in a committed relationship by returning to something idealized, uber-romantic and deliciously tragic. It sounds immature and short-sighted, but we need that crap sometimes. We're not going to get it from our current set-up — which is no doubt good, but oppressively vanilla and functional.
Also like HPV, SBS will often pass on its own, leaving both partners largely untraumatized. You write some journal entries, confide in a good friend and pen a few emails that never make it out of the outbox. It only becomes dangerous when you start engaging in high-risk behavior — leaving fantasyland and actively initiating contact with the person. You're in this advanced stage of SBS right now.
If you want to live up to your name, Compassionate Dumper, you're going to have to decide (and soon) which side of the flirt fence you fall on. If you're going to stick with your girlfriend, you've got to stop talking to this woman; no more meeting for tête-à-têtes, no more foreign-language postcards. If you don't, it's going to be a hell of a lot harder to keep up the pretense of commitment and the lies will flow more and more. But the way I see it, breaking up with your girlfriend is a no-fail proposition. You're making yourself happy by one, making it possible for your ex to get back together with you (although she already dumped you once — why will she want you now?), and two, opening yourself up to finding a new lady. You're also making your girlfriend happy. She deserves better than all this wishy-washiness. It'll be hard to let go, but you're doing her and her screaming ovaries an invaluable favor if you do.
 

I slept with my best male friend a couple times in high school. It wasn't a big deal. I saw him occasionally when I was home from college. I recently moved home and one night we slept together again. Neither of us were drunk. We watched a movie, held hands, laughed non-stop, and then I stayed over.

Now things have changed completely. He barely talks to me when we hang out in our group of friends and breaks plans every time we make them. His closest friends say that this is how he treats all girls now and that I shouldn't take it personally. I really like him as a friend and I thought I might like him as more because of our past and how much fun we have together. I'm not just some girl he brought home from the bar. Why is he acting like this? And does this mean anything more than friendship with this guy is off the table? — Repeat Offender