Oh you are definitely not alone, I would think for most people motivation comes and goes in waves.
For example sometimes I cannot bring myself to do reviews for weeks.
If you have a particular goal, it should help to focus on just that goal, and not spread yourself too thin. Is it to have a simple conversation? Is it to pass a test? Is it to listen to a specific show or read a specific manga? Regardless of which one it is, you can probably find a way to cut out whole areas of vocabulary that are not relevant at your stage, and decrease your load.
I think drops in motivation come from excessive stress, from treating learning as a job, and piling up more and more overtime and overdue “job” obligations. Why not step back and allow yourself to do only 5-15 minutes of reviews (job), and then do something enjoyable like playing a game in japanese, browse a manga or watch a video. Of course it’s more difficult in the beginning when most material feels overwhelming but… hang in there!
Incidentally the “turning learning into a job” aspect is my gripe with tools with a fixed path, like WK, Duo, you name it. “You have to do this specific lesson even if this topic is not part of your goals and interests” is an approach I don’t understand. Japanese is not a career, right, just let me pick and choose.