So i'm a relatively attractive guy, and don't have any problem landing myself multiple plates around HB7 status. A couple months ago, I met this HB9 through a friend, and went into an LTR with her, (18, she's 17). Our relationship has been relatively stable, with her only pulling a hamster every couple of months. Let's call my LTR L. Recently, I was at a party with some friends, and a couple of girls that go to L's school came up to me. one of them clearly having a few rounds beforehand, started getting really touchy with me, and asked me to go upstairs with her. I declined, telling her I had a girlfriend, and was just there to wingman for my friend (already getting some). The sober one of the two asked about my girlfriend, so I started telling her about L, because fuck it's not like I have something to lose here. After giving a brief description of L, the girl says something along the lines of "Really? I thought she was with Chad Thundercock." Me, clearly being confused I asked what would lead her to think that, and she gave me a long explanation about how L hangs out with this Chad at school, and on a couple occasions were at other parties together putting on quite the show.
Now, considering the social status of L, and the fact that most of her friends form the largest cock carousel in history, this didn't surprise me. After the party, I texted her and asked her about this, and she went full hamster on me, giving me the works on how she didn't mean it to sound that way, and how she didn't think about it as cheating on me, etc. So, since I haven't completely downed the red pill yet, I get pissed off and just hang up in the middle of her sob story. I don't know what to do at this point, because I still like this girl, but I damn well know she's going to do it again. Fast forward a couple of months, I'm at a concert with a really close friend group of mine. While there, I run into this girl I was friends with in sophomore year, who had completely transformed into an HB8. We get flirty, and I start thinking about what happened with L before. This plate at the concert was clearly wanting some, and as I was getting more pissed about what happened with L, I said fuck it, and ended up making out with this girl. Over the next couple of months, L starts getting more attached, and her social life slows down (doesn't go to as many parties, stops hanging out with the Chads from before, actually starts being more loyal). Now I've read enough to know that shit is probably still going on with L, and I hear little things about her here and there, but nothing like it used to be. She still flirts with other guys, but how can anyone expect to bed an HB9 and not have her attention whoring continue? At this point I'm completely done hearing about her doing stupid shit like this, and give up on being a loyal beta bitch. When I see hot girls and i'm out, I hit them up, get their contact info and just practice being completely alpha with them. I have at least 4 plates now all crawling like cave creatures for my dick, and could easily become the Chad we all aspire to be. The thing is, I've never fucked another girl while dating L, and I still respect her enough to not go full Thundercock. I want to keep this relationship going with L, but she's a whore and I get my share of pussy.

TLDR; Dating an HB9, kind of a slut. I'm a beta and like LTRs. I don't want to break up with HB9, so I cheat on her on the side sometimes as well. I know she's doing stuff on the side, but she doesn't know I am. She won't go for a non-exclusive relationship, but I don't want to break up with her. What do I do?