Boss can't take no for an answer

I'm a trainee and my boss is a pain, he keeps changing my days and than when I had to put my toddler at a nursery to manage new days assigned I took a few Thursdays off to settle my child. Just cause he doesn't have anyone else, he wants me to work half day. Though i told him I can't. He just take no for an answer.

What do I say to him? I need this job for another 6 months and than I'm done.

Every day at work I have panic attack or I'm this close to having one. The thought of going to work stresses me.