"Hi guys. Not going to get into the the whole break up. That would be a novel. This is more of a give your thoughts and experiences for others. I'll start with my thoughts and a quick recap of what I went through."
After a break up time is needed to heal yourself. Some people need more and some people need less. After long enough you'll begin to cope with the fact you lost something whether it was important or not, loved, your world, or just a warm hole that you grew comfortable with. You have to reflect on what "it" was that you really lost.
Time will pass and hopefully you've done what Cody said if you want her back. His post is 100% pure gold go read it if you haven't. Many times though, as much as you may not be able to comprehend it at the moment, you'll come to the realization that it was an acceptable loss. You're going to have to get the idea out of your head that you can make things right. Because you can't. Your relationship is dead and gone. That doesn't mean you may never have a second chance at a new one with them.
Even after all of the healing that takes place after a break up, regardless of a good or bad break, there may come the day you run into them again. It's a small world after all. Guess who they have with them? If it's only been a short amount of time it might be the rebound or who she broke up with you to be with. If its been a long time it may be their new boyfriend or in rare cases, the guy she broke up with you to be with. In the case of a long time, if you use the advice this forum has to offer, you should have someone new in YOUR life. The two of you aren't in a relationship anymore. Regardless, when this happens it may dredge up those old feeling and even reset all of the healing you've done.
This is the time to be a man and suck it up though. Don't get beta and undo all the healing and improvements you've worked so hard on. Don't let the fact that she moved on not let you do the same. Break ups can be hard. They can drain you spiritually and emotionally. The truth is when you do see her again with another man you WILL feel something. Just don't act upon it unless it was your brother or father. Those are probably the only two people that would know they deserved it and not call the cops on you. If it was your best friend then at least you know now that he wasn't a friend. Other guys won't give a shit.
If you're still strung up on getting your ex back you're going to have a ton of thoughts. How should you act? Be cool? Be a dick? Act as if you don't even see them? Unfortunately I can't answer that. Not even sure myself. I would have to say act like the person you are. Unless you're a violent person. Then tone it down. My personal advice though, which may be right or wrong, I'm just a dude like you, would be to act like someone comfortable in their own skin. You own it. If your gut says ignore them then do that. If you want to test the waters and see what happens then go ahead and say hi.
Afterwards go back into healing if needed. Cry like a bitch. Hit a punching bag. Go bang your new girl. Don't call, txt, facebook, contact her period in any way directly afterward unless it's something like my situation and you absolutely have to. If you did say hi you'll be able to gauge the waters and you'll know if there's still attraction as long as you gave yourself time to heal. DO NOT let yourself be fooled by the thought that she still loves you and wants to be with you. Unless you killed her puppy, part of her will feel that way but it doesn't mean she's giving up what she has now.
You have to learn there is going to be loss in your life. Shit happens every day to good people. Even if you're not a good person shit happens and it probably happened in your relationship because you're reading this. Or you're bored and wanted something to read.
I'll open up. My situation was we broke up. It was bad too. The reason was I simply wasn't around enough because of work. I knew there was a problem so I quit my job and told her "I quit" via txt. Her response, "me too". Can't blame her. Only after the break did I find out from her son's gf that this guy played her and filled her head full of bf destroying techniques. Powerful ones at that.
I went into NC and healed. I stopped going to mutual friends and focused on me, like you should be if this just happened to you. After enough time to improve and heal I returned to a pool hall/bar where the two of us have mutual friends. I was told "she won't be here for a long time" by the owner and sadly believed him. Sure enough she came in with the new guy.
For some reason women will shock you with how many steps down they go to fill a void in their life. Don't be shocked, I was. Sure enough I slowly lost count of my beverages and BOOM. The last thing I remember is 3=0. I got black out. I lost my car keys, a single flip flop and a silver dollar. I'm pissed about that flip flop. Regardless I needed my keys.
The next day I was informed that my ex and her new bf drove me home. Someone let it slip at my house, I'd been having a hard time and they told me they could see the color drain out of her face as she realized she had hurt me and that I could only be hurt like that if I did care. Ungh, Guys, don't get black out if you see your old lady with a new man.
Everyone from the pool hall said I behaved great and was actually the life of the party. I am comfortable in my skin and I hope you are enough to be the same in a situation like that. So reluctantly the txt had to go out. Amazingly she was nice enough to inform me that they were going to a concert. How sweet of her. Again I txt'd and asked about my keys. Then I cracked a joke asking if she just invited me to a concert.
Guys, Heed Cody's advice. Be non-reactive. She's someone else's problem. If you do anything stupid it'll only hurt your chances. Be the bigger man