Relationships aren’t black and white, so your relationship with your female friend is going to ebb and flow and may end, and then may even start up again — there’s no set of rules for friends the way there is for family. It sounds like she really betrayed you by dating your now ex-boyfriend, and while you’ve remained friends, I trust that the friendship has changed as a result. Forgiving is good, forgetting, not always a smart idea.
If the guy you’re with now is the one that you’re going to commit your life to, then that is the relationship that has to trump all others. You can choose not to see your female friend for now because she’s causing too much chaos in your relationship with your boyfriend, or, if it’s manageable, you can see her, but with boundaries. The boundaries can include seeing her less often, and/or only talking about things that don’t include your boyfriend and/or your relationship with him — you can be creative about how you life your life.
I hope that helps. Let me know how things go, and please follow me @AskAprilcom on Twitter. 🙂