I started working on another project - then I realized my original project is still worth working on, and regained some energy to get back into it. I honestly felt pretty unmotivated for 3-4 weeks. I think that was burnout.

Sounds about right, and I've experienced that back-and-forth switching a decent amount. When I start getting stuck in loops like that unable to decide what to work on, I feel like it's my brain telling me to step away for a while. So I usually try to do something away from a screen. Even better if it's constructive, as it'll usually help you either solve a problem you're stuck on or help a decision or two that you might have been torn between.

Personally my burnout-preventing hobbies include biking, walking my dog, reading a book, woodworking, geocaching, and photography.

I always get to know when I get closed to getting burn out. Take a day off, go hangout with friends or do some outdoorsy stuff. If you actually keep on working and actually get burned out, it will take you longer to recover. A day or two off won't help, you will need longer.

This burned out feeling is like mold growing up. The more it manages to grow, longer it will take to clean up.

To keep it away, have regular breaks, go out, take your special someone out on a date, just chill w/ good music etc.