I want to say that ultimately only YOU can know what is best for your situation, because ... we cannot know the full picture through Internet forums. You gotta make the final decision yourself.
But ... come on. She's sleeping w/ someone else. You still want to be her best friend? She's castrated you. She doesn't see you as husband material. Harsh, I know. But I'm in the same boat: my W doesn't see me as a H no more, and I don't want to be seen that way by her. No fn way.
Me: 50, MLC/WW 45 Young kids Nov 2015: BD1 Apr 2016: BD2 Jan 2017: W filed Feb 2017: D final