AITA for cheating on my cheating girlfriend after she repeatedly betrayed me?

Okay, brace yourselves. This is a real messy one, and I’m not sure if I’m the asshole or not. So here goes:

I (M19) have been dating my girlfriend (F18) for about a year and a half. Everything seemed fine, but about 3 months ago, I found out she had been talking to other guys behind my back. And when I say talking, I mean FLIRTING with them and sending very suggestive texts. I didn’t even go snooping, the messages just popped up on her phone while we were hanging out.

Naturally, I confronted her. After some massive denial, she admitted to texting multiple guys, but kept saying she hadn’t done anything physical with them. She swore she’d stop, promised to change, and begged me not to break up with her. Like an idiot, I decided to give her another chance, thinking I could trust her again.

Welp, fast forward a couple of weeks and I catch her red-handed. She had been meeting up with one of these guys, sneaking around behind my back. This time, it was even worse because she flat-out lied to my face about it and tried to gaslight me into thinking I was “just insecure.” But it was obvious. I was DONE. So I broke up with her on the spot. And she threw the biggest tantrum, calling me everything from “controlling” to “immature.” It was a nightmare.

Here’s where it gets juicy: After the breakup, I’m pissed, heartbroken, and frustrated. So I go out with my friends to forget about her and just blow off some steam. We end up at a party, and that's where I meet Sapphire (not her real name). She’s cool, she’s funny, and we hit it off immediately. I’m not thinking straight because I’m still angry and hurt from everything, but one thing leads to another, and we end up hooking up that night. No regrets.

But here’s the bombshell: my ex finds out about Sapphire (yes, she knows about the hookup). She flips out. Like, full-on meltdown. She’s texting me, calling me a "hypocrite," saying I “betrayed” her even though she was the one cheating on me first. She claims I was “just as bad as her” and says I should’ve never gotten with someone else so soon, even though we were technically still in a relationship when I hooked up with Sapphire. Apparently, I "disrespected" her by doing it, even though she was out there being shady as hell.

Now I’m seriously torn. On one hand, I ended things and was emotionally checked out way before I got with Sapphire. But on the other hand, I did hook up with someone else so quickly after the breakup. AITA for moving on that fast and “cheating” on her, or is she just pissed because I got over her and found someone else faster than she thought I would?