Why do we all stay?!

I'm new to the forum but recognize shades of my domestic partner in all of your postings. He has the inattentive form of ADHD - the kind where he is completely uncommunicative. Like everyone else the relationship started out amazing - he was sweet, kind , thoughtful, I was fooled into thinking he was the kind of man I could marry. Of course, the second we moved in together he became a completely different person. He never even told me he has ADHD until I figured it out and confronted him. Even then he refuses to discuss it. He is a healthcare provider  but refuses to treat his condition with anything other than supplements.

My relationship is incredibly lonely in that ADHD is such an odd condition that my friends don't believe it's a "real" thing and I am just pathetically making excuses for him. I have read everything I could to try to understand that he has a kind of disability - to feel compassion for him. But He refuses to have any conversations with me about anything negative and will just leave the room and turn up the TV to drown out my crying.

He does all of the things that truly make me wonder if he's just simply a selfish, lazy, liar. Like many in the forum he has been caught many times online pursuing other women to the point I don't even look and wonder anymore, I just assume it's on-going, He lies constantly to avoid conflict or to avoid taking responsibility for his actions. I can't even get a straight answer out of him if he's paid the cable bill!nhe treats other people just as badly and doesn't seem to have any moral compass when it comes to the "right thing to do"

I receive little to no affection -he won't hold my hand of Kiss me on the mouth. He will dismiss my feelings or concerns. It's a lot like living with a resentful teenager where everything is a chore, annoyance or imposition on him.

There are positive things about him and ways that he enriches my life. But mostly I feel lonely and dismissed at best - angry, exhausted and resentful at worst.

it is gratifying to know that I'm not the only person who is going through this - but honestly why does anyone stay in these relationships?! 

I question myself every day and feel a deep shame.