This is one of those times when it’s best to say nothing. Because your kissing another guy while you and your boyfriend were having trouble, is not the same thing as sleeping with someone else, and because you are pregnant and your boyfriend isn’t stable enough to handle your possible admission of kissing another guy when things were rocky with the two of you, I’d suggest you not say anything to him.
You have a baby coming, and you need to focus on providing a stable home for the child. When kids are involved, they have to come first, and adults have to start acting like adults and that means making tough decisions. I know that you feel guilty because you did your boyfriend wrong by kissing another guy. There are other ways to repent for having wronged him. You can commit yourself to being the most wonderful girlfriend, wife and mother in the world. You can bend over backwards to support him when things are tough — emotionally and otherwise. You can hold your tongue in the future when he does something you don’t like.
If you weren’t having a baby, I’d suggest otherwise, but the rules change when kids are involved.
I hope that that helps.