"I absolutely feel responsible for him. He is literally my most troublesome child. He has no friends of his own, doesn't speak to his family. He sounds like an absolute dead beat, but appears as a professional man, social, to all intents and purposes "normal"
But he's so troubled. I know he's suffering from depression, he refuses to see a doctor. We both know he's addicted to gambling, he refuses to see GA. The burden is all on me. To "support" him".
You have a choice re him; your children certainly do not.
You are not responsible for him and you do not have to support him but you certainly seem to be co-dependent in this relationship and that state is unhealthy. It is not your fault he is the way he is and you cannot fix or rescue him here. If he refuses to see a GP about depression you cannot make him go. Anyway how much of this is due to depression and how much of this is actually due to him being an abusive arse with a gambling addiction?.
"We are definitely giving the children the impression this is normal. But most of the time we do have a happy, normal, family life. I try to shield them as best I can but he uses the children as a weapon to get to me whenever I threaten to leave as he knows they are my world. As I'm typing this all I'm thinking is that all responses will say HE'S A FUCKING COCK, LEAVE HIM. And he is. Sometimes. Others he's not, he's my best friend"
I think you are kidding yourself big time when you state that you do have a normal, happy family life. It reads to me like the seven circles of hell.
Is this really the impression of "normality" you want to give them; they will not thank you for staying with their gambler dad if you were to choose to and will wonder of you why you put him before them. That could well destroy your future relationship with your adult children because they will go onto despise you as much as their dad for bringing such misery upon them from childhood.
If this person is "your best friend" I would hate to meet any of your enemies. Do you think real friends treat each other like this and besides which he is not just your friend, he's your H currently.
"Our youngest is a girl and when I went back to him earlier in the year, my mum sat me down (they don't know he's been physical in the past, only verbal. And not all of it) and said "I understand why you're going back, you have children. But don't let your daughter grow up thinking that being treated like this is normal"
Going back because of the children was never a good reason or justification for actually going back. You should have left after the verbal violence towards you (that is domestic abuse as well) but did not. Your mother's second sentence is certainly correct, do you want your DD to end up marrying an inveterate gamble like your H?. You're doing your fair bit here re showing them damaging lessons on relationships and you cannot protect them from the realities of his addiction. They see you and your reactions both spoken and unspoken to him all too clearly.
"I know I have to leave. But I'm a coward. I'm so scared"
Feel the fear and do it anyway. You've been a coward long enough and that state has got you precisely nowhere. Get proper legal advice now and start protecting yourself properly before he bankrupts you all and trashes your life completely. I would also suggest you now also talk to Womens Aid with regards to the abuse on 0808 2000 247.