It’s possible you are going too fast, or in other words ‘rushing’ yourself? I once tried to go fast but started to get burnt out and then hit a wall. I ended up resetting to a previous level (made a post After a long absence, I reset (to level 7), and I’m glad I did), and then decided to focus on enjoying learning Japanese, rather than playing the ‘fastest to level 60’ game. I found a thread with like-minded folks at 
Let’s Durtle the Scenic Route 
, and now I focus more on making steady, enjoyable progress.
Haven’t had any issues with burnout since then! Even been able to handle a recent 3 week absence (and forgetting to turn on vacation mode), leaving me with 700-ish reviews. Just worked away at them bit by bit (and stopped doing new lessons until I got things back to normal), and since I’m not pressuring myself, it’s still actually fun.
…
Your case reminds me a lot of how I felt the second time I hit a wall and came back to a bunch of reviews (over 9000! 2000!). I ended up choosing to reset to a previous level (see link above for how I made that decision), and if you’re experiencing similar frustration, I highly recommend at least considering whether or not a reset to a previous level would help. Many people report positive experiences when resetting. Something to think about! 