UpAbove, love, people have not been harsh, they have been direct.
WHEN you are out of this situation, you too will ask yourself the same questions. We'll all be here to hold your hand and tell you that you did what you could at the time.
Your reply to AF was heartfelt, but you need, seriously to think about what you have said.
I stay because I don't know any different. He was my first boyfriend
OK, but you know NOW that the way he is treating you is unacceptable, it is a deal breaker and you deserve better. You have told him you won't accept it. He still won't stop. He doesn't care enough about his family to want to change.
He is your FIRST HUSBAND, your second will be better 
I stay because I have horrible unremitting depression and would not cope on my own.
Erm, I'm willing to bet that the frustration of living with someone who is investing his time and affections in others, yet rubbishing them to you (while STILL devoting more time to them than you) is a major factor in your depression. Get rid of HIM and it's highly possible that your symptoms may alleviate.
I stay because we have a child who is severely autistic. There are plenty of people that raise children, even autistic ones alone. You may find that his behaviour (being more structured in access visits) will be more help to you than the current situation. i.e you may get respite when he comes to collect his child.
I stay because basically the idea of being on my own is fucking terrifying
I think the most terrifying thought in the world is a lifetime of living with a man that doesn't care, that places emotional affairs over his wife and family, that even when faced with the serious consequences of his emotional betrayal, STILL won't stop, THAT is terrifying. Yet you condemn yourself to a life sentence with this creature, when you could get shot, you could eventually find another partner, one who WOULD step up, who WOULD put you and your child first.
You need to sit this cretin down and read him the riot act and TELL him that you will be better off without him... If that doesn't focus his mind, then tbh, you will have done yourself a favour getting rid of him anyway. Long term you will see that.