Me and my boyfriend broke up and I kissed someone. Now we’re back together and I feel awful about kissing someone else but I’m afraid he would leave me if he found out. What do I do?

You broke up, meaning you terminated the relationship, so there is nothing wrong about kissing someone else if you ended a relationship with someone. He shouldn't get upset about it and if he did, it would be a good time to consider if you want to be with that person. It's understandable that you feel awful about kissing someone else but you weren't together anymore. If he leaves you for that, then don't worry because there are a lot of fish in the sea and you will find the right person for you. It takes time but eventually you will.

Be honest with him. One of the most valuable things in a relationship often times is the honesty and trust. You can tell him how you felt after you broke up, why you did that and clarify him, that he is more important to you than the other person, you kissed. In general, I would recommend you two, to have a deep talk about what made you break up, and how you can do better now, since what you don't want, is to break up again. From personal experience I can say, that such a talk does not result in a heavy argument, but in a lovely understanding of each other.