Working TOO much. How can I reduce my hours?

This job has gotten extremely stressful for me these past few months, and I find myself coming closer and closer to quitting. But since I don't want to leave impulsively, I've decided to reduce my hours instead. I've always received far too many hours for my liking as a part-time floor associate (~5 days a week, 6-8 hours), likely because my manager knows I'm a remote college student who can work any day at any time. They're always packing me full of hours and I'm done with it.

I took an availability form from the break room, but I'm not sure how to fill it out. It asks for specific days and times that I'm available/not available, but in general I just want to reduce my overall hours. Would it be better to just talk to my manager directly? I don't even know what to tell her because she's very nosy and was constantly asking me when I'd be done with my online classes so I could work more...