advice please

So I just found out that my SO slept with his "best friend from college" years ago. A part of me knew this happened because of their body language when they take pictures together. I know it's in the past, and honestly I don't care about how many people he's been with. It's different with his "best friend from college" because they had a connection before they slept together. What I need advice on is how to get over it fast because I look at her and I become almost hateful inside and I don't like feeling that way. Please don't be rude.