We all have a few levels where something doesn’t stick no matter what we do, life is overwhelming at that moment, one simply doesn’t feel motivated. As long as you stick with it, you will move forward. A step forward in the right direction 
We are normal humans who make a lot of mistakes, take inexplicably stupid decisions and we learn from them. That is our strength. So, it’s okay to cut ourselves some slack when things don’t work out.
I am not saying do not be frustrated, the fact that you are frustrated means you yearn to progress and that you are making efforts to move in the right direction. I am suggesting a “reframe”.
Whenever I am in a slump, I always imagine myself as a farmer in a field (since I say to myself this is Kanji farming). Cultivation is a delicate process.
You put in too much water, crops die (too many lessons in a short time)
You do not put in enough water, crops die (too few reviews)
You need to have certain space between the seeds, otherwise all the seeds will die due to lack of nutrients (enough spacing between lessons and reviews)
You will have weeds, so need to take extra effort to remove them (leeches, need to put in extra work)
For some reason, thinking like this calms me down when I make too many mistakes. I understand that not all seeds will live (I will forget some items), I understand that some seeds grow easily (items we are familiar with or emotionally connected to), I understand some seeds will have a hard time (leeches). It is all part and parcel of the process. I prefer progress over perfection.
Sorry for the long answer, this perspective helps me. You can try this or something else that makes an emotional connection with you (maybe a sport, maybe cooking etc.). Wherever we look, we have parallels.
頑張れ birdee san 