Most Helpful Girl
I've been there. But he wasn't just talking to other girls, I got dumped for another girl. It killed me. But this is what you need to do.
-Think about all the BAD TIMES. Completely erase the good times from your memory. Remember that those good times you have with her... they weren't special. You could have those times with ANY other girl. They were NOT special and you will have BETTER times with better people! Whenever you think of a good memory, just think to yourself "yeah that happened, but then she did THIS" and just think about all the bad things she did. Get angry.
-Go out and talk to other girls. You don't need to date any of them, you just need the attention. You need to force yourself to realize that there are other girls out there, hotter girls, better girls, nicer girls. There are literally millions of girls in the world and this one is SO insignificant. Go out and check girls out, talk to a few, develop a crush. It'll be good for you.
-Don't make contact with her. Don't do it. Also, keep yourself busy. You'll find that whenver your bored and not doing anything, that's when you'll think about her the most. Stay busy, and find a pretty girl that you can focus your thoughts on instead.
That's what I did except the girl version lol and I was over him quite quick :)