I have cheated on my boyfriend?

It's best to just be completely honest with him and to never commit adultery again. If you are serious about each other and love each other you will be able to work passed it, if you can't you know it's not meant to be, and you can move on and have the time to improve yourself in other ways and improving yourself will be more satisfying and fulfilling and improving yourself will give you a better chance of meeting the person that will fit you as a person perfectly. Honesty is always a better option even if you hurt for a little while in the beginning.