Juxtapositions, Your boyfriend did break your trust. So it is natural for you to be very upset. It will take you time to forgive him. From his point of view, he thougth you'd never find out and guys will be guys. Guys have a greater sex drive and given an opportunity it's hard for them to resist. But it might not have meant more than just sex. It's possible for two people to have sex just for the sake of sex. Maybe he loves you, but just wanted to fulfill his sexual desire on the side as well thinking you'll never find out. Since he's your first one as you have said, it will be very difficult for you to forget him. I'm sure it will be difficult for him to forget you as well. The best thing to do is to try to work it out if he has realized it's his fault and if he is sorry about it. The best relationships are those that can survive the challanges such as the one you are having. You do have to be careful that he doesn't take you for a sucker and does it to you again. You have to make that absolutely clear. One reason why guys might do that is if the girl is very devoted to him. Make him earn his love too sometimes.