What I did: should I tell him I cheated with a girl?

I have a suggestion. If you chose to tell him, please word it differently. Because if you tell me you were ‘forced’ to cheat on me, there had better either have been a gun involved or some sort of weapon. In which case I dont think it counts as cheating. You were not forced to do anything. So lets admit at least to yourself, that you willingly cheated. Now, I’m not being judgmental here. People make mistakes. You have to decide. Because if you cant put this aside, then it’s going to ruin your relationship anyways. And so maybe it would be better to come clean and try to salvage it, then to slowly have it burn out and he has no clue why that happened. I know a lot of people believe honesty is the best policy. But I guess that depends on the person. If someone cheated on me, they knew they’d never do it again, and things were able to continue being ok. I wouldnt want to know. But a lot of people dont see it that way. This is really going to have to be your decision.

I think you should be honest and deal with the consequences. You need to look at why you really did this. Attention? Too weak? You need to focus on fixing yourself, too.