AITA for confronting my girlfriend and her best friend after finding out they slept together while I was in the same bed?

Edit: with Full Update <----

I've been dating my girlfriend for almost 3 years. We live together in a small apartment, and things were mostly great until last weekend when my entire world crashed down around me.

For context, her best friend from college has been staying with us for the past week. Let's just say she's always been flirty with both of us, making suggestive comments, but I brushed it off as her personality. My girlfriend always laughed it off too, saying "that's just how she is."

Friday night, we all went out drinking. Heavy drinking. We bar-hopped until 2 AM, and I was definitely the most intoxicated of the three of us. When we got home, I remember collapsing into bed while they stayed up talking in the living room. I passed out almost immediately.

Here's where shit gets messed up. I woke up around 4 AM to use the bathroom and noticed they were both in bed with me. I was on one edge, my girlfriend in the middle, and her friend on the other side. I didn't think much of it, figured they just crashed, and went back to sleep.

The next morning I checked my phone while they were still sleeping and saw that my girlfriend had accidentally sent me a text that was clearly meant for her friend. It said: "Last night was incredible. I've never felt that way before. I can't believe we did that with him right there."

My stomach dropped. I scrolled up and saw no previous texts in this thread, meaning she must have deleted their conversation history and accidentally sent this new message to me instead of her friend.

When they woke up, I acted normal, made coffee, and waited until her friend left to go shopping. Then I confronted my girlfriend. At first she denied everything, then tried to gaslight me saying I was crazy and paranoid. But when I showed her the text, she broke down.

She admitted they'd fooled around while I was passed out in the SAME BED. She swore it was just a drunk mistake, that she's not gay or bi, just "curious," and it meant nothing. She begged me not to tell anyone and said they both feel terrible about it.

I lost it. I packed a bag and went to stay with my brother. She's been blowing up my phone saying I'm overreacting, that technically she didn't cheat because it wasn't with another guy, and that I should be "flattered" they were comfortable enough to "explore" with me there. Her friend even texted me saying I'm being a jerk for making my girlfriend feel bad about "discovering herself."

My brother thinks I should just break up with her and move on, but we share an apartment lease, most of our furniture, and even a cat. I don't know what to do.

AITA for feeling betrayed and refusing to talk to either of them? Am I overreacting to what they're calling a "harmless drunk experiment"? I feel like I'm going crazy with them trying to spin this like I'm the problem for being upset about what they did literally inches away from me while I was unconscious.