To put things into perspective, I (M 20) and my GF (F 21) had been dating/together for four and a bit years. Typical high school sweethearts, we wanted to move away and start our own lives after college.

Ok great, so here we have a man who rather than spinning plates during his youth has settled with one girl. Now bluepill me from a few years ago would say this was true love. That such a relationship must be solid as most relationships during highschool don't last that long. These two must be very deeply in love.

This isn't the case, this man didn't bother going after other girls due to his gf being a buffer against rejection. Basically if he dumped her, then he'd have to go out in the harsh dating game, he could just be comfortable with this girl and stay with her instead.

We decide to spice things up a bit and “open” up our relationship a little. It was pretty much a join decision but looking back she was kind of insistent on the whole “sex is fun” idea.

So basically she's getting ideas now. Ideas of branch swinging. She wants out buddy. So why doesn't she just cheat? Simply put, her hamster will let her rationalize that this isn't cheating as he agreed! So what if he says no then? Then clearly she's isn't going to look for other men!

Real desire cannot be negotiated

If he said no, her hamster would spin. She'd cheat, but spin it in her favour saying that her bf was controlling, he was controlling her sexuality by saying no to the open relationship. So she just had to!

She is on a night out with some friends when she phones me and asks if the whole open relationship thing is still cool, explaining she’s met a guy (M 24) and asks for me to pay for a hotel because she wasn’t working at the time. I agreed. I tell her to go for it, and she does. She texts me to say she had a good night and that all was well.

Now OP you might as well have dumped her at this point. She didn't ask your permission if the open relationship thing was cool. How do I know? Because she met and spoke to this guy before your phone call. This guy seduced your gf before the phone call. Your gf if she was unsure about the open relationship thing, she'd have told you before she went clubbing. She rang you for money. You are literally being a beta bux at this point!

The week after, she mentions she’s meeting up with this guy again. Wasn’t expecting it but I’m cool with it. Again, asks for me to pay for her hotel. A decision I would live to regret. She doesn’t text me the entire night. Weird, but okay maybe she’s just having a good time?

The dick was good. She's wanting a second sample. She only told you so that she has money for the hotel. She'd not ask the guy, because that would make her look low value to the guy.

The next day I receive a load of messages about how she needs a “break” from “everything” right now. Meeting this guy has apparently opened her eyes. A little more conversation and we’re over. She’s completely cut me out of her life. I check her snap chat story, and of course, she’s with him. I’ve heard from her friends that they’re dating now.

You my friend have been branch swung now. Hypergamy is a bitch. The signs were there from earlier, you decided to ignore them because of fear of losing a partner. Just as women are excellent at rationalizing, we men are excellent at denying. You agreed to this open relationship thing because you were scared she'd be lost. Oneitis.

By the way. The fact they're dating within a week? Doesn't that suggest she knew him from before?

And on that bombshell, keep lifting!