How can my boyfriend forgive me when I can't forgive myself?

I have been in a relationship with my boyfriend for about six years now. In the past, our relationship was difficult and frustrating. We argued a lot, and due to that, there was a lot of tension between us. We stayed together because we love each other and wanted to make it work. I used to party a lot, and several times I got into situations where I would end up kissing someone else. These situations were never more than just kissing. I have come clean about these situations with my boyfriend, and he decided to forgive and move forward with me. I love him so much and want to work things out too, but I'm having a difficult time understanding how he can forgive me. I can't seem to forgive myself. I'm overwhelmed with feelings of guilt and unworthiness.