Too good to leave, too bad to stay?

I'm new here - stumbled across this forum when I was looking for ideas for living with a dependent spouse. 
I live with my husband who has ADD, depression, alcohol use and anxiety. We also have a child, who is struggling with anxiety, and probably has ADD too. I'm the glue that keeps our family together. I sometimes describe my life as having a 7 year old, and a grumpy petulant teenager who can't organise himself or anyone else, gets angry about not being able to find things, has no frustration tolerance, no problem solving skills and sulks if I try to have an adult discussion about how we can make things work better. 

I wonder why I stay. I think it's primarily because I know that my husband wouldn't cope without me. We've been together for 20 years and I've always kept everything running. I gave up my full-time job because I knew that our family wouldn't survive if we were both working full-time. I also felt like I couldn't leave while he was depressed but it's been over 10 years now, and I don't think I can do it anymore. I've asked him to leave a few times but he always refuses to go and it would be way too unsettling for our son if I moved out.

Apologies for the rant. It has made me realise that I do need to leave - it just takes a lot of courage to do that, and I'm worried about how our son will cope. I also worry about how my husband will cope and whether he will completely fall apart.

Would love to hear positive stories from people who have left a difficult ADHD relationship and where it's been positive for everyone involved or tips on helping children of parents with ADD.