If your main cause of burnout is too many reviews, and low accuracy, my #1 main advice to avoid burnout is to sloooow right down with Lessons.
Instead of 15 per day, maybe slow down to 5 per day. Heck, if you’re really struggling, I’d even suggest put a hold on doing any new lessons for a while until that sense of overwhelm and dread starts to ease up again. Then, start re-introducing new Lessons once you’re feeling good again.
I also like the (informal) thread-group 
Let’s Durtle the Scenic Route 
, which is focused on finding a comfortable and sustainable pace for doing lessons and reviews, without worrying about going ‘fast’ or ‘efficiently’. In other words, to get back to a state of enjoying studying Japanese again, and avoiding burn-out.