I was in this same predicament. Same length of relationship and time since my partner had sexual contact with her best friend.

Except after learning that I let my anxiety and trust get bad. It was not made any better by the fact that we were long distance. If I have anything to suggest it’s to not let this piece of knowledge affect your trust in her.

Buuut, she might just rebound to him if you guys ever break up / are in a rough situation cause that’s how my relationship ended :)

Basically in my case, I let my trust issues get the best of me. She broke up with me, and then went to her best friend despite the constant reassurance that they would never get together and that their past experience was “nothing to worry about”. Essentially proving my worries as valid.