Thank you all so so very much for all your feedback and support!
I'm feeling a lot better today, still heartbroken and confused... I sat and talked with her alone, and yes, she does feel like he cheated on her with me, she also feels like he put me first. She says she doesn't blame me for any of it because I was lied to as well. She understands that I in no way engaged in this to hurt her.
It is going to take some time, and I am so angry at him that he lied to us both, I feel like he threw what we had or could have had away.
We have agreed that the ex is no longer welcomed into our home. She herself is in a monogamous relationship, and her partner has no clue about anything, not to mention she came in and disrespected us in our home by doing what she did under our roof.
GalaGirl... Thank you for the reply, that in itself opened my eyes up tremendously!
Taking this one day at a time, but I hope we can mend our relationship, because I really don't want to leave, and I really don't want to be with anyone else either