'Expat, how do you suggest that I work more? Who will be there for my kids after school?'
They are not babies. And you can pick up another job on weekends or during the day.
I can see however, that you don't want to.
I wish you luck, because this man won't divorce his wife and even if he does, he won't marry you.
And your relationship is rocky.
People have told you how precarious your situation is. This man can kick you out on a whim. Your 14 hours wouldn't do for even a tax credit claim, and if you're in Highland Council, you'll be put on UC and expected to seek out FT work unless your children get so much DLA you qualify for CA and even then, there will be expectation of obtaining more work. They won't give a toss if you want to be there for such old children after school.
The only major thing you can do to improve that, and that is in your power, is to work more so that you have more money, greater earning potential in the event of a split.
Carrying on as you are knowing how bad your financial situation is so you can be there after school for older children, whilst all the while knowing their situation could be utter rot if this man dies or throws you out.