How to set boundaries at work?

I work full-time at my job, and I totally understand that sometimes things happen and I might run late when it comes to clocking out. However, this is a daily occurrence with one if my supervisors. She will give me a task, and then when I’m almost done and about to wrap things up to go home, she’ll give me more work. So what would be me leaving 20-30 minutes later would be 1.5-2.5 hours later. She overworks me and is comfortable doing so. I know some employees volunteer to stay, but that has nothing to do with me. I want to leave around the time I’m scheduled to leave. Again, if this was once or twice every now and then, I wouldn’t mind, but this happens everyday almost. It’s irritating and I have trouble sleeping because of it. I already work overnight. How do I get it to stop without seeming lazy and/or coming off as an a-hole. I just got a raise so finding a job that pays as well will be extremely hard (I’m working a entry-level job recent college grad).
How to set boundaries at work?
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