How do I stay motivated with repetitive tasks as an undergraduate research assistant? In a nutshell, I'm drawing circles on images for most of my day (more precisely, ROIs around cellular components in fluorescence imaging). I volunteer to do this on weekdays from 10am to 7pm, and I stay late until 9pm once or twice a week to finish the day's set of images when it's considerably large.

I'm spending at least 45 hours per week drawing circles. I'm working efficiently (it takes me on average of 15 minutes per page of stitched images), I try not to take breaks to maintain momentum unless I'm noticeably slowing down (and limit them to 10 minutes), and I try not to take more than 30 minutes for lunch.

I understand that monotonous tasks are a key part of research, and it's suitable work for me as an undergraduate (as they're low-risk and require little experience). I'm also lucky to do research so early in my career. But in the evenings, I begin to think of all the other undergraduates out there getting ahead doing amazing work (like Martin McLaughlin, who already produced publishable, original work in his first year), and I feel discouraged and expendable.

And it's entirely my fault, too. If I spent my time in high school more effectively by learning Python coding for automation, statistics, and understanding more of the relevant literature, I could be so much more useful to my supervisor doing more stimulating complex tasks.

It's been a month since I've started, and the long hours of this repetitive task are wearing me down. But I don't want to let my supervisor down, so I intend to keep getting the daily image set done each day. She hasn't required me to set these hours or get the image set done each day, but I've been doing this for so long that it's expected (though she's always very appreciative each time I submit, and she works longer hours than me).

I plan to continue until the project is complete when my supervisor graduates in December (lessening hours to the evenings when classes resume). Until then and for future research opportunities, how can I keep myself motivated with monotonous tasks and avoid burning out?
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.