You have a number of options:
1. move to a better paid instructing job where your experience will be better rewarded. (Difficult in the current climate, but not impossible.)
2. supplement your income with another job, and do less instructional hours.
3. get a better paid job doing some other sort of flying.
4. get a well paid job outside of flying.
I think you have considered all these and would much rather your employer just paid you some more, because you don't want to leave, and you enjoy instructing (to a degree), and you hate being taken advantage of. We all get ourselves into situations that are not ideal, and most of us have difficulty extracating ourselves from them, you are not alone believe me.
If I were you (and the relationship with my employer was okay), I would explain the financial situation to them, so that they can see that something has to change. Get them on your side - they may be able to help in a number of ways. More money
, agreeing to a changing to part-time instructing to allow you to get a part-time job outside of flying, contacts with other aviation organisations so that you can get a better paid job in aviation, a good reference or even just some feedback on your 'operating style' that may allow you to chose a job outside aviation where you will succeed and be happy. Your leaving will be a problem for them too- at least if you involve them, they will have had a chance to do what they can. They may be able to do very little of course...
Best wishes with your difficult decision.