When me and my ex dated the first time we both lost our virginity to each other. when we broke up he slept with alot of other girls. during the break up i would think about him with other girls every so often and get upset but i could kind of distract myself. Now that we are together again the thoughts are even more constant and i cant escape them. its just constant. i know hes with me now and i shouldnt worry about it. i just get so upset when i think about him being with someone else. sometimes it doesnt feel the same because of it.
I feel your pain! My guy and I are back together as well but the thought of him with that rebound girl is enough to make me sick to my stomach! (I actually am still a virgin!) My only advice to you is to seriously try to distract yourself. As long as all of that is in the past, leave it in the past. Don’t let it ruin what you have going for you now. It’s so much easier said than done, I know, but when these feelings bother you do not bring it up to him because for him they are old news. Talk about it here or with a friend that gives good advice. That’s what keeps me sane!
And if it’s confidence/comparing yourself to them that you’re having issues with, do things that make you feel pretty! Take a long bubble bath with candles, take care of your skin, hair, and teeth, go to the gym, eat healthy… anything you can do that makes you feel like you blow all those other girls out of the water with how gorgeous you are! 
I know rebound chick can’t hold a candle to me, and I’m sure it’s the same for you! He’d be with them if he liked them better, and he’s with you
thank you ill try harder with distracting myself. like i know none of the other girls compare to me because they all were cheating on him and whores. im not like that at all. it makes me sick to my stomach to think hes been with other girls. im afraid of him getting bored of me. i need like little things that we can do together. i never know what to do when im around him.
Rebounds are nothing more than a shitty attempt at getting what you and him had. He was lonely and too scared to be on his own so instead of dealing with the break up by himself, he tried to find someone else to find the void that you left. When he got with these girls he was probably trying to speed things along to the level that you and he were at so that he could feel comfortable again, if they really meant anything more to him than he wouldn’t have chosen to come back to you. He obviously feels comfortable with you enough and missed what you two shared enough to come back.
You should talk to him about how you feel, if he’s serious then he’ll listen to what you have to say. Build you’re relationship on trust. As typical and over said as that sounds its the truth. If not you’ll hold little things against him forever and if you ever have an argument you may end up randomly blurting out something that has nothing to do with the situation in an attempt to gain the upper hand. Don’t let it come to this. Honesty shows compassion and trust.