Resolved Short novel: FL cheating with boyfriend’s friend out of spite

I forgot the name of a short novel I read about a FL who finds out her boyfriend is cheating on her and sleeps with his friend from college out of anger. They both keep cheating until the boyfriend realizes that she probably found out and decides to propose in front of all their friends (which she rejects because she likes the friend). Then it turns out that the friend liked her the whole time and the boyfriend knew that in college and chose to chase her and use her to show off.

 

Maybe it's After My Boyfriend Cheated on Me

 

Thank you!