Thank you for the advice :) His friend doesn't actually live near him or anything; she lives in a completely different continent than he and I do. Yet I still get these stupid feelings because I know they communicate online as much as he and I have. Sometimes I can't help wondering about the things they talk about. I have a close guy friend myself, so I feel like my jealousy is unfair in this case, but I don't think my friend and I are as close as my SO and his friend are.

I have expressed my envy before (for a completely different situation), and he knows I've been cheated on. He was completely understanding then, but since this girl is his best friend, I don't want him to think that I'm trying to tear them apart from each other or anything. I don't even want to do that because 1) they're best friends, and 2) it wouldn't be fair. As far as I know, he's not jealous of anyone I talk to. I don't want to seem like a negative person either and bring his mood down.

But you do have a point, so I'll try to communicate as much as I can about this without sounding so petty. All this sadness and anger has been eating me up inside, and I don't want to feel like this anymore.