Mish is 100% right, I went through exactly what he described to an absolute tee.
Its a terrible feeling when you have to basically leave someone when you still love them with all your heart. You're going to be torn up for a while after it but trust me, no one will look down on you for it.
I had one person tell me I was wrong in what I did out of probably the fifty that asked me what happened. That person came back and said "now that I see who she truly is you had every right to leave and you deserve better." Those were the exact words out of this womans mouth. Just be confident in your decision and know that everyone who cares about you will stand behind you and probably have decided long before you had that it should be over.