I cheated on my boyfriend with my ex what should I do?

I was with my ex for 4 years, broke up with him because I wasn't in love with him. Started dating a new guy, a much nicer, sweeter guy. I wanted to continue being friends with my ex but at first that didn't happen. Well now we started talking, I went to a party at his house, and next thing you know we hooked up. I regretted it the second it was happening and stopped it. Now I feel so guilty, I am 25 I have never cheated, I don't even lie. I believe in karma, now I don't know what to do, I have no feelings for my ex, but I feel awful. I don't want to tell the guy I'm seeing because I don't want to hurt his feelings.