Read DB and DR, I'd recommend both. Starsky is absolutely right that you need to be strong about your boundaries in your relationship. If you are OK with an open relationship, that's your business. If you are not, then be crystal clear about that with her. I wouldn't recommend demanding and throwing a tantrum, but be calm, respectful and clear about what will and will not work for you. There's a great post out there (someone help me with who posted it) about "dropping the rope" when dealing with a wayward spouse.
As counterintuitive as it sounds, you need to drop any attachment you have to the outcome of getting her back. If you don't, you will become needy, possessive and angry, and those are things that will drive your W away. Spouses respond to strength. This is something that took me a long time to learn, as Starsky can attest; but once I did, that's when her behavior and rhetoric started to change. It's still a work in progress, but I feel like I'm finally on the right track.
Ex Rzrback Me 43 Her 44 D11, D15 T21, M19 BD 9/9/2014 Piecing
Sometimes if you want to see a change for the better, you have to take things into your own hands - Clint Eastwood