I feel really guilty about what i did?

It sounds like you really do have feelings for our boy friend now. If he feels the same way he will forgive you. Just tell him when my bfncheated in me he didn't tell me and I learned it from someone else. That hurt allot. Tell ur bf that you regret it so much and you are telling him bc you care about him and want to be with him. If it is mention to be he will understand.