"I don’t know how I’m going to manage to continue to live like this until the youngest finishes his A-Levels".
You won't and your children and you will all be destroyed from the inside out long before that day arrives, if it ever did. If that day did arrive what then for you?. You then announce you want to separate and otherwise divorce?. Waiting for the children to go off to college or university and then divorcing may make the kids feel guilty that their parents sacrificed their own happiness for them. We owe our children much more than the physicality of an intact family. We owe them our truth.
Do not be afraid to move on with your life now and take your own responsibility for happiness.
What do you want to teach your children about relationships and what are they learning here?. It's a shedload of damaging lessons they will then carry over into their own adult relationships. Lessons that they learnt from you two as their parents.
It’s not what we say, but what we do that matters. Telling our children they deserve healthy, respectful, and loving partnerships isn’t taken to heart if we don’t have the courage to live up to our own words. What we model for them is very much what we might expect for them in their future relationships. From this perspective we might question the sincerity of the expression “for the sake of the children.” Living in mediocrity or worse burdens children with very confusing messages about relationships and happiness. It certainly instructs them that loving marriages and partnerships are not their birthright.
You have a choice re this man and they do not. Do not remain in such a marriage for what are really weak and or otherwise spurious reasons; no obstacle is ultimately insurmountable and financial concerns/equity are NO reason or basis to remain within an already dead and loveless marriage. Is that what you're going to tell your kids; that you stayed with their selfish and self absorbed father because of your concerns surrounding equity?. They will call you daft for staying and wonder also of you why you put him before them.
Its not "easier" for you to stay and you also need legal advice re all aspects of separation and divorce. Divorce is not failure OP: living in unhappiness is.