Why can't I let go of my husband when I left him? He is moving on and it is killing me.

I have been unhappy in my marriage for quite some time. My dad just died March 29. I left my husband a month later by leaving the state and moving in with my step-mom. Now I am thinking I have never dealt with the grief of losing my dad and made a rash decision to leave my husband. He has a new “friend” he met on Facebook now and claims to be just friends. She is over at our house often and he says they are not having sex and I don’t believe him and I am a wreck about it. My neighbor saw her backing out of my garage yesterday at noon. We both assume she spent the night…who wouldn’t? I am being told by my husband that it is none of my business if he has a friend because I left him. WTF? It’s still my house too. He jumped into a friendship after I’ve been gone 2 months? I am crying all of the time and just can’t move on. Any suggestions?