Why am I struggling so much with this?

CPA04

H cheated on me with g/f my entire pregnancy, left me for her, is still with her, and all his parenting time includes her.  I know it's how it's going to be going forward and have to accept it since H's selfish self sees zero wrong with it, but I'm having such a hard time with it.  I hate, hate, HATE having someone else play the mother role to my baby. I know B will always know that I am his mother, but I hate thinking of all three of them "playing house", sleeping in the same room together, hell, for all I know, she's the one who was getting up in the middle of the night with him in the past.  The whole thing just makes me want to throw up.

On a convoluted brighter note, I signed the papers on Thursday and they should be filed Tuesday--and I feel good about that.  God willing, H will only have supervised visits and B won't have to be the next victim to his bipolar, sociopathic father's alcoholism.