Hey! So I'm 18, and I have a lover (he/him for reference purposes) from a different university. That's the label we chose haha but we say partner sometimes to friends to avoid the inevitable (even if it's lighthearted) teasing. He actually was my best friend first, then turned into a friend with benefits, then turned into my lover. We're not exclusive, but I asked him not to sleep with one girl specifically about a month ago (she's had a crush on him a few times before, she's his best friend at his university, and I asked him to not sleep with her because there's a good chance she'll crush on him again and for a few other reasons; he also wasn't planning on sleeping with her again anyways so I don't believe it was hard for him to agree). This past weekend, he gets insanely drunk and...sleeps with her. He was texting me both before and after and he didn't know what he had done wrong and turns out he had forgotten that he agreed to not sleep with her. He's been insanely regretful and in all honesty it's not something i'm willing to end our situation over because we aren't really exclusive anyways, but it hurts knowing he crossed a boundary. It honestly makes me feel like...inadequate because it feels like he's prioritizing her over me? Even though I know that's not the case and i've talked with him about this.
What do I do? What does he do? We've been fine since and he knows I'm still upset but he doesn't really know what to do to help, while I have no clue what to do about the situation in general because I don't want to sacrifice myself for the relationship but I don't think this is drastic enough for me to consider ending the relationship.