Need advice desperately 😭😩

My boyfriend of 2.5 years and I split 6 weeks ago, just before my 21st birthday. He went and slept with a girl that I work with a week after we split, celebrated her 21st birthday with her and met up with her 4 times after supposedly having sex once...(which I don't believe at all, I think it was more than once). We then met a few days ago whilst out, he was telling me how he made a mistake leaving me and we decided to give things another go. I then found out about him having sex with the girl I work with, who is his best friends sister and boss's niece, so she is always going to be in his life and also mine at work! She has been texting him saying how he should have let her know that he got back with me and has been telling people that she thought she would get a relationship out of their endeavour. I haven't had a chance to speak to her and hear her side of the story due to working different hours and ensuring that I remain 100% professional!

I now feel I can't move on and that he's betrayed me, I didn't sleep with anyone else as I was heartbroken we had ended. I know he was single, but if it was with a stranger who I never had to see again it would be so much easier to get over! I need advice please as I feel like I'm losing my mind!!! 😭😭😭😭😭