I cheated on my partner with my consensual fwb. I regret everything.

As the title says, I cheated. It started out with casual fwb stuff with my best friend. No feelings attached. My boyfriend wanted me to have a sexual outlet while he was busy. Then my best friend started talking about how lonely she was.

The scariest part of all of this is my boyfriend says I was getting more romantical with my bsf after she said she was lonely because I was "trying to be a good person and that he might of done the same." I talked to him a bit more about this response, and he says it's not him diminishing what I did or sweeping it under the rug. I can't help but feel like it is.

I am so extremely depressed after this. I lost my best friend, my friend group, and my boyfriend's trust. I have never felt so much guilt. I have been stuck bedrotting because of this. I'm scared that our relationship won't be fixable because he doesn't see how severe this is.

How do I fix things with him? How do I stop thinking about my bsf after I went no contact? I think about her while doing anything, and I can't stop.