I'm not going to tell a guy I'm seeing that I slept with his close friend
Why don't you want to come clean?
I don't want him to think we're a couple
Sleeping around when you're seeing someone is never justified. You need to tell him the truth before he finds out himself, which will cause even more pain.
But I like him and I care about him.
Then you have to choose which one you're going to start a relationship with. You can't just have the cake and eat it too.