firstly, I totally understand about not being able to cope; this is stress FOR YOU.
Secondly, do you have other friends (face to face) to just take some weight with you on this? What about his friends? YOU are doing nothing wrong, but the mind is complicated as you know.
Thirdly for a man, being skint, not being the provider etc. is a real pride issue.
Being prevented from seeeing his kids? Well his ex is a silly c*w. What a selfish act on her part. She's certainly not thinking of the children and exacting some revenge with them as pawns (I have been through this myself with my ex and his ex; yes complicated i know!). My husband had to recently admit to being £160k in debt; after he advised me to quit my job....My job was as an MD and I was the major wage earner (£108k basic salary). Strangely it now appears I was actually paying his debts and his ex-wife but didn't know it!
Imagine the discussions at home
Anyway, I have REALLY thought long and hard about my marriage. TBH the stress of his ex wife, debts, kids etc. is A LOT OF STRESS for me...to the point I wanted a divorce.
He now has psoraisis in his hair, I have seen him cry, he is DEPRESSED. We have to sell the house to pay his debts (I have already given him £25k cash and now my £60k in this house to help (we're doing an IVA).
If you are really concerned about him (given his past) you should ring a help line (after all that's how most sections are done; not that he necessarily needs one, just that you can't seem to make him get help). You can certainly take some verbal abuse from him, but if he did anything worse, well...?
Of course the final option is to cut your ties for your own health...sorry but that IS an option.
Vent as much as you can.