No one ever said polyamory was for the faint-hearted.
My word of advice is this: if your primary loves you, and you love her as much--and if she's aware that her actions are making your relationship with the new girlfriend strained, then she needs to back off from your girlfriend and realize that the likelihood is this: you will always be a vee, and not a triad.
However, at the same time, your post betrays something deeper--you mention issues in the primary relationship, and you say you want to become monogamous with the new girlfriend. Is this still NRE talking? Because that's not usually what it says.
The vibe I'm getting from this is that maybe, polyamory just isn't for you. It's not for everybody. What's most important right now is for you to sort out your feelings on the matter first. Do you really want an open relationship? Do you want your primary to seek her needs (with a woman) elsewhere entirely, or just separate from your relationship? Does new girlfriend meet all the needs your primary does for you? Can you meet her needs while still maintaining the primary relationship?
These questions are what you need to concentrate on at this time, because the bottom line is this: if you're acting without being certain of what you want, you're going to end up being unfair to both of these women who love you--and that's got to be the last thing you want.
Be well. <3