If you can't maintain your own interest, at that level, you will never be able to maintain the interest of a company.
You have always to be convinced that you are a irreplaceable asset in what you do. In this sense, when you walk, you willingly hurt your employer. That's not really acceptable.
Tell him that you are working more hours than you started with and that you will fulfill your obligations.
When he keeps to try to make you go, do not comply in that idea, but just block it off and say that you don't understand what he wants you to do.
Think of arguments that present you in a good light and don't be afraid to personally take responsibility for doing your work and to assume credit where it is due.
You can picture it this way:
A school dropout vs. a person who failed to graduate are a person who is a bum vs a person who tried everything he could.
There is work for people who failed at even being mediocre in one regard, but there ain't any jobs for bums.
