Cheated on boyfriend with bestie.. help!

I've been with my boyfriend for about a year. He treats me nicely but I found my feelings grew quicker than him which resulted in a ton of rejecting while dating. Between that and the fact that he doesn't like to talk, I found myself confiding in my best guy friend of 10+ years more than normal. I met at my bffs house with another friend for drinks after they'd finished bottomless brunch mimosas. His friend left for work and we went from making out to having sex. I haven't told my boyfriend and I told my best friend we can't be friends after this and haven't spoken since. Should I tell my boyfriend even though I vow to stay monogamous? I feel so guilty but don't wanna break his heart. We're almost 30, and i feel like I can fix this.