I'm not going to be a better employee
Why don't you want to be a better employee?
So that I can avoid working more hours. My boss is often too critical towards me.
Your boss won't be the only thing that's affected by your work ethic. You should try to be a good employee because it will help the other people around you and your clients. The company will benefit from it as well.
I'm not going to do my job if they make me work more hours.
It can be hard to work more hours when you are already having a hard time but you cannot stop doing your job. If you do a better job you might get a better position with better hours.