I am truly sorry you were betrayed! Just remember - just because you are his sub does not mean you have to accept his mistress as a metamour or sex partner. He cheated on you. Don't let him walk all over you. Sub does not equal doormat. He will have to rebuild trust first.
The world opens up... when you do.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein