gubby
What do you get out of this relationship now?.
Re an earlier comment of yours:-
"I stay for the children"
No, you stay for you, not them. Do not bring them into this. You're too afraid of the unknown to leave but no obstacle is however insurmountable. The above thinking is a huge mistake on your part. That teaches the children a lie, its a terrible burden to place on any child.
What d'you think they are going to say to you; "thanks mum"?. No they will not. They will call you daft for staying and worst still wonder why you put him before them. They will hate you for doing that to them and will see you as weak.
You really want them to potentially repeat all this in their future relationships; you're showing them that currently all this to you from your H, their dad, is acceptable to you. Your children see and hear most of what is happening at home between you and their dad and perhaps worst of all may even blame themselves for their parents troubles.
Do not leave them this as their legacy.
Is this really what you want to show them about relationships; that people really do behave like this in marriage?. Surely not. This is his 4th extra marital dalliance since last November, stop kidding yourself here by believing him at all.
Love and put your own self first for a change rather than him.