What is wrong with me?

Im a 23 y.o. male in the Boston area. As a disclaimer, I am going to give a few details before I describe what happened. I'm not a bad person, but I used to cheat. I was cheated on during my first long term relationship and since then I sort of lost the need for fidelity. I'm not saying I openly sought out other women to cheat on, just drunk mistakes happened more often then not.

The girl in question was part of the first relationship where I thought I did everything right. I didn't cheat, I was completely honest, and more importantly I had no problem doing either. It was honestly the best relationship I had ever been in, and I've had about six long term relationships since the age of 16.

We dated for a year, until this January. We got in a small fight on new years because I had to go home for an emergency and she was upset about it. Then she went on a skiing trip for a week, came back, and broke up with me.

I am no stranger to breakups. I've been the bad guy plenty of times, and I've been on the receiving end as well. The thing that caught me most about this one was her apathy. To this day, I still don't know why she left me. When she actually broke up with me, she came over, said she wasn't happy, left, and that was that. There were no preliminary signs of her not being happy, no warnings, nothing. I'm not ignorant either; if she was not doing well, I absolutely would have noticed and done whatever necessary to change it.

Long story short, that night I got drunk and angry. I sent her an email (nothing threatening), just asking all the questions that bothered me. Occasionally she would answer, but it was always the same: "I wasn't happy, I'm sorry, I need to not be with you for a while to get over this". As months past, I got drunk and sent her emails (again, nothing threatening). Not every weekend, but maybe once or twice a month, just asking the same questions. Sometimes she would answer, but as times passed the responses thinned out until eventually I got "stop contacting me or I will call the police". I'm not a crazy person, nor did I think it was worth it to keep occasionally prodding her for the police to get involved, so I stopped. I think she overreacted a bit, considering none of my emails were never in any way threatening or violent, or even mean. Regardless, I stopped.

It has been six months. I have had flings, hookups, the whole lot. I perceive myself to be a pretty strong person mentally: I had severe anxiety and did pretty well to control it until medication finally kicked in. I don't usually show too much emotion amongst my friends simply because its usually me who they come to when they have issues in their own personal relationships. I am always brutally honest with them, and more importantly I don't let my own emotions or biases get in the way; not because I don't care, but because I can usually just blot out my own issues.

Last night, I was at a bar in town, and she was there with her new boyfriend. Words cannot describe the epic crumble that I displayed to everyone around me. My chest tightened up, I couldn't be in the same room, and I was just extremely upset. However, I knew if I left, it would just seem like a sign of weakness. I wasn't trying to be macho or anything, but I always tell my friends I'm fine when it comes to this stuff, mostly because I usually am fine. But last night I could have cried. After half an hour of pretending, I just left.

First of all, why the hell do I feel this way after six months. I loved her but she wasn't "the one". Is it because my ego was hurt when I saw her leaving me even though I thought everything was fine? Is it because I still have no answers to any of my questions? Or do I simply just miss her, and it only shows when I am put in situations close to her? Is this karma for everything I've done before her?

I am lost. I fell apart, everyone saw it, and I am just disappointed with myself. Six months of gradual recovery down the drain, as I sit here typing out a diatribe of questions. Where the hell do I go from here?