Asking Cheddar to put things on hold with Gouda doesn't help anything, but you have every right to be hurt, and absolutely need to have that discussion.
The issue isn't that Cheddar has a new partner and thus you have a new metamour, the issue is that you took things slowly with Swiss due to Cheddar needing time and space to process it, only to then not be given the same consideration or opportunity for discussion from her. And that's what needs to be talked about and addressed.
Asking her to put things on hold won't fix the problem of you not being considered or consulted, all it does is turn it in to a petty revenge thing, and create more problems (as you've seen).
Give her your blessing to keep things going with Gouda, but also, let her know how much she hurt you when she didn't give you the same consideration she asked you to give her. If she takes that onboard and treats it seriously, then you'll get through this. If she is dismissive of that, then the issue is in your relationship with her, not in her relationship with Gouda
That sounds like my kind of world