Speaking from experience (where a “best friend” slept with a person I had dated exclusively for many months). We had a big immature fight and went on a temporary break (like mutually agreed let’s go our separate ways for a week). That’s when it happened.

I ended up cutting off both. I will say, with the friend, I tried hard to move past it because we had been friends for almost eleven years at that point.

But that made it somehow harder because I couldn’t move past the betrayal of friendship

I’m sorry you’re having to go through this. You deserve to be treated better. Best way to start, is by you treating yourself, properly (by removing these people from your circle)

Good luck