I can't leave as my kids would be absolutely devastated.
Yes they would, and then they would get over it just like millions of people get over things that change. They will cope with change and will thrive. It isn't them that has the fear of making a different future - you are seeing this from your perspective, not theirs.
I'm prepared to work at the marriage and myself to make things better.
There is such a danger in giving in to this temptation to take the moral high ground as The Good Person and make yourself feel better that way. That way lies co-dependency. It will destroy both of you. You will totally lose who you are. It is so, so seductive.
I sent him a long message last night to outline how I feel. I know it sounds ridiculous to message him when he's just upstairs but I wanted to make sure I got my point across without a row. I wanted to make sure my words were fair and balanced. I outlined how we both need to change and I included what I thought I needed to do.
Well, you can try to get him, the arsehole, to agree that he is being an arsehole. If you tell him how you feel he may indeed be smitten by conscience and thoroughly repent - who knows?
You can try to pretend that in fact, you are equally responsible and to blame for him being an arsehole in order to placate him & encourage him to go along with the myth that you can both change and that you, the fair and balanced Good Person will be doing your bit of changing because that is what truly Good People Do, and if he doesn't agree, well perhaps you need to be even more fair and understanding and try even harder, eh? Might you not show him how you will reward him and yourself in this way? -Please, please be very clear what you are inviting him to participate in here.
Or you can draw your line in the sand and say it ends here.