First of all, I just want to say that this sounds very familiar to me – I also struggle with ups and downs of ‘motivation’, and have been for my entire life. It took until roughly age 30 to finally get a hint about what may be the root of this issue, and in my case it turns out that I had undiagnosed ADHD, which i never in a million years would have guessed if someone hadn’t pointed out the possibility to me.
So, I obviously don’t know anything about you, but just from that description I’d like to suggest to you the possibility that maybe you might have an undiagnosed, or maybe untreated, condition such as ADHD or maybe something else. Or, I could be jumping the gun and there’s no such condition. [Note: There are many conditions which also have similar symptoms, and the only way to know for sure is to get a proper diagnosis from a qualified mental health professional.]
So, you may be interested to check out a couple of threads that are related to mental health (Improving our mental health together 
) and specifically ADHD (The ADHD Thread – this one happens to be archived, no longer accepting new comments, but it’s got lots of good info about ADHD anyway).
As for how to avoid burnout in general, I would say that the main thing is pacing, specifically pacing your number of Reviews per week by adjusting how many new Lessons you do. Try to find a level of doing Reviews every day such that you can not only maintain this pace on a good week, where you have lots of motivation and nothing else going on, but so that you can maintain it even on a bad week, when your motivation might be low, and you’ve got other random things that might come up.
If you’re able to find that kind of a consistent, sustainable pace where you can maintain it even in a bad week, then you’ll very likely be able to avoid burnout.
Myself, I even found a thread-group for folks who prefer to take the ‘slow and steady’ route, 
Let’s Durtle the Scenic Route 
. There’s lots of stuff in that thread about how people are pacing themselves. And I’ve even posted some of the little techniques I’ve come up with to avoid burnout as well. It’s an ongoing thread, and anyone can join in, just an unofficial ‘group’.