You should let it go. You did nothing wrong.

I don't really understand why you feel so guilty. The guy dumped you and told you to move on, and you did--in a big way. Good for you. You didn't cheat on him; he broke it off and you were single again, and free to do whatever you wanted. Just because you two got back together later, that doesn't mean it was wrong of you to sleep with others in the interim. Again--you were single the second he ended the relationship.

Guilt is overrated. Yes, it can be instructional and help you learn from your mistakes, but so many people take it up like a cross and can't ever put it down. Others feel it for no good reason at all, either out of habit or because blaming themselves can be easier than acknowledging bad behavior in someone they love.

I'm wondering if you went out purposely looking for guys to sleep with after your boyfriend dumped you, and that's why you feel guilty. Maybe you only had sex with those dudes to punish your ex, and now you two are back together and it's killing you. Even if that is the case, so what? I still say your guilt is unnecessary. You were hurt and angry, and rightly so. You're certainly not the first person to leap into someone else's bed to get back at an ex. In a situation like that, it's hard to distinguish good motives from bad ones, and pointless to try.

What happened happened. He dumped you and you got laid. Big whup. No harm, no foul. Let it go and move on.

If you're wondering whether or not you should tell him, that's up to you. I wouldn't--remember, you were single at the time, and it's really none of his business. But if he asks and you don't want to lie, then tell him the truth. If he doesn't like it, too frickin' bad, he broke up with you. What did he expect you to do, sit at home polishing your chastity belt?