You might hate this, and it's only my opinion, but:
Be with her 100% or 0%.
If you're anywhere in between, you're going to continue to be a wreck. If her being with you and you alone isn't an option (hanging out with friends doesn't apply) and she still wants the option to fuck other dudes, your best choice FOR YOURSELF is 0%.
Can't remember where I read it, but it's very, very true: Always be willing to walk away.
It'll hurt like balls, but it will be better than dragging yourself through hell until you can't take it anymore and end up breaking it off anyway.
If you had started the relationship open like that, and everything built from there...I would say go ahead. But for a relationship to start as a monogamous relationship and then venture into less and less dedication? That is only a terrible sign. It is time to break away
[size=20:7c51ae7520][color=red:7c51ae7520]email is a better option: email@example.com