I just saw a similar thread below, so I figured why not post to get some advice. I haven't talked to many people about this problem because everyone seems to not want to hear annoying relationship issues.
So the story is my girlfriend and I (of one year) decided to take a break back in mid October because I decided to cram for the Dec. LSAT. We had planned all along to get back together, but I supposedly made one mistake (although I'm not sure if I consider it a mistake, or more like a betrayal on her part). I forgot to lay the ground rules for our "break" because I just assumed that since she "really loved" me, she would just take it easy for a month and a half.
Little did I know, she bumped heads with this guy that we both sparingly knew about 2 weeks later and apparently hit it off. I didn't want to say anything to her and she was very open about their little "relationship" with me. I didn't want to be the "controlling" boyfriend and not allow her to see him (because at that point I realized we didn't exactly enforce any strict groundrules for our break), so I decided to just play it cool. I figured they would go out a couple times and then be done. Come on now, you really can't blame me for not setting any groundrules. We were really serious and had made future plans, the whole deal.
Unfortunately, I was wrong. I guess pretty soon afterwards they become pretty serious and pretty much started living together.

At that point, I just had enough. It was the hardest blow I had ever taken in my life (because me and this girl were very serious) and felt like I was being used as her puppet or something and decided to completely severe all ties with her. This was around mid Novovember and I haven't spoken to her since (although she has tried a few times to email and call me, but I haven't replied).
Anyway, even today I am still confused about the situation. I had never gone through something like that before and it really hit me hard. I've been depressed ever since and feel like a loser a lot because it seems I got the worst of luck. I don't know if I necessarily blame her 100% because I know that our love was very serious (there's no doubt in my mind about that) and also, we didn't lay the groundrules for our break. I just assumed that she would wait a month and a half for me because it would be the considerate thing to do since we had been seeing each other for so long and had become so close. She decided to do otherwise, though and now it's history.
What do you guys make of this whole situation? The way I see is it, she did betray me and stabbed me in the heart because she knew that we had planned to get back together in a month and how much it would hurt me seeing her with another guy. I really don't see how it can be argued otherwise, but maybe I'm not just seeing it right. Did I just get a bad hand, or is this girl a serious bitc%?
Thanks for reading and any help! I don't really want to discuss this problem with friends because they would just be annoyed. Hopefully you guys can make this whole situation for me a bit more clear because I have been thinking about it for nearly 4 months now and still feel really bad about losing my girlfriend/best friend, especially THIS way.