How do I make it up to my boyfriend?

I'm slept with someone when my bf and I broke up. I'm saying I HAD no IDEA WE WERE GOING TO GET BACK TOGETHER OKAY???? But I still felt bad when we did... So I decided to tell him today and he started to cry... I've never ever in my life felt so bad in my life! My chest felt heavy! I couldn't breath.. Every time he sniffed in I felt a little bit smaller. Every time he swore I felt like I deserve the worse words possible. He said he forgives me and I'm lucky he actually loves me cuz he wouldn't ended it.