How could he had done this ? exboyfriend now helpp me pleasee?

DON'T SETTLE FOR FRIENDS WITH BENEFITS! Guys are crazy creatures and you will learn this throughout your relationships, the more serious you get with a guy the more you will learn to hate the male species (and love them at the same time). I think that if you've heard this #1 girl is a slut that he's just in it to get some from her. Don't let him play you! Be done with him for good. As for you not being able to stop thinking about him, it will get better in time. Soon you'll find something else to think about and he'll just be a memory, maybe you'll even find a better guy that will occupy your mind. I know you think he is the greatest guy ever right now and you don't want to lose him but he's probably talking to other girls trying to get with them (cuz that's what guys do) but still trying to keep you because your a good girl. Find a better guy and don't settle.