How do I stay motivated with repetitive tasks as an undergraduate research assistant?
First of all, I think you're taking the wrong approach here. Drawing circles isn't helping you at all in any way. You can't put on your resume that you know how to draw circles. You aren't getting paid. You aren't learning anything. You're burning yourself out by working 45 hours a week and you haven't even started university yet.

In my opinion this is a perfect opportunity to learn programming and image processing. I have worked with many undergraduate and even high school students (I was one myself) in different labs, and I can tell you that you're perfectly capable of learning basic programming skills over the next two months if you work at it. With these skills you can attempt to make software that recognizes these ROIs in your images automatically.

From the sound of it your supervisor is a graduate student and not a professor, and since you're volunteering, they should have no problem with you taking a different approach to solve the problem of identifying ROIs. If they want people to do simple repetitive work, they can use a service like Amazon Mechanical Turk.

So to answer your question, academia is all about coming up with new approaches to solve problems or learn new things. If you want to stay motivated, use this time to learn something new that will help you in the future, and also help your supervisor.