What do you get out of this relationship now, what is in this still for you?. I have to look at your roles in all this as well, you've seemingly been playing the usual roles of enabler, provoker (because you never forget) and adjuster (because you keep on adjusting to all the crises that happen). This is not emotionally healthy or stable is it?. You're lurching from one crisis to another. His promises to change are empty ones.
Is he really a great dad or H?. Bet your kids do not think he is a great dad at all and wonder of you why you and he are still together. Women in poor relationships often write such guff when they themselves can think of nothing positive to write about their man. As is the case here. You are far too over invested and completely under qualified to help him, not that he wants your help or support in any case.
It sounds like he has long term addictions to both over the counter medication and alcohol. Alcohol as well is a depressant and if he is taking this with tablets its really bad news. He is using such things to self medicate his underlying emotional problems. He has run away from problems since you've known him and he is really out of control now. He could quite easily overdose and end up in hospital, that is not beyond the realms of possibility here.
None of you have ever been considered in all this by him, this is all about him.
You have been profoundly affected by him in terms of your work and yourself being stressed due to your H's addictions. The harm here being done to your children through seeing their dad like this (and you have really not been able to fully protect them from the realities of their father's addictions) are incalculable (its already affected their exam results and they have seen your reactions to same) and will probably stay with them for many more years and into their own adult relationships. Your own relationship with them going forward could be profoundly affected for the worse too. It may well pass that they will not want to come back home to see either of you very often when they leave home (which will be sooner rather than later too).
What you have tried to date has not worked. You are likely codependent yourself in relationships which has also cost you dearly. You cannot rescue and or save someone like your H; he has to want to address the root causes of his addictions properly and he is showing no signs of doing so. He may never do so either.
How many people in your own social circles know the full extent of his addictions; probably very few. I would start talking to his GP particularly if you and he go to the same practice and seek help too for your own self from the likes of Al-anon. This thrives on secrecy and ultimately you can only help your own self. You cannot rescue and or save someone who ultimately does not want to be saved. There are no guarantees here; he could still choose to use substances to blot out everything even after losing you and everyone else around him.
You have a choice re this man, your children do not. Make good choices now for you and your children. Would you be prepared to seek legal advice (which you do not have to immediately act on) with a view to divorcing him, after all his addictions come first and he is in a relationship with those. You are all well down his priority list even if you are on it and you are not.