You poor thing, you must feel almost at breaking point with it all.
I think you need to stop thinking about him for a while and think about you and your kids. You cannot make him make good choices for you all , but you can and must stop him dragging you all down in the process.
Think long and hard about whether you can stay together, both short and longer term. For what it's worth, in your situation the drug use would be a deal breaker for me, I could not have anyone using drugs in my kids' home, and especially not my husband. I would ask him to leave temporarily to give you space to work out what YOU want to do next. It is then up to him to work out what he does.
And then I would, as has been said, contact the mortgage company, CAB and benefits advice, whilst also searching for any feasible fulltime job. Call in every favour and contact you might have, and fling yourself on people's mercy with regards to the mortgage company etc. Explain fully the situation you as a mother have been left in by your husband's actions. Escalate higher and higher if you can until you find someone in each company who will listen and help.
I am so sorry he has done this to you. I am quite sure you will get through it, you sound capable, hardworking and strong... but I am so sorry you have to go through it in the meantime. Do keep posting here at each stage, the wealth of knowledge on here is incredible.
Keep going, OP. Keep your head above water. Rooting for you.