Girlfriend won’t stop fucking with other guy

I find that a bit difficult to answer because a lot of context is missing.
“…we decided to try cuckolding… doesn’t exactly sound like a well-considered act. Why did you decide to do this? What else have you done? Swinging? BDSM? Voyeurism? ….
Well, the question for me isn’t what you should do, but what you can still do. Pandora’s box is open; what was locked inside is simply coming out. No matter if You like it. In my opinion, this damn box is opened far too often just for fun without even the slightest precaution as to how the contents should be handled…. But if that weren’t the case, then it wouldn’t be Pandora’s book but a labeled shoebox.
No matter who opened the box, you can’t close it again. You’ve talked about it, but you don’t trust their word—and not for the first time, and probably not without reason? So why keep blathering on, keep letting yourself be lied to? There are only two things left you can do: Either you breakup, now. Or you can continue to enjoy the good things and see how it develops and whether you can or want to handle it.
I would do the latter. Right now, you only have a lot to throw away, but very little left to lose. It’s quite possible that your girlfriend will get fed up with the whole thing and pull herself together again. And the thing with the bull… that wouldn’t be the first time a woman thinks this is her great new love, only to then realize – oh wonder, he’s a good fucker, but my loving heart is waiting at home. There are roles for cuckolds and bulls, and a woman can easily get lost in them. Especially when they get really turned on by something, they react no differently than horny men: thoughtlessly, headlong. Maybe you’re already practicing mentally taking her back without much instruction, without accusations? Remember, you wrote “We” decided. You were part of the decision. So: wait and see how it develops and enjoy what you can enjoy about it (and can’t change anyway).