Almost everyone new to Japanese has the same problem eventually. I think it’s mostly because your brain isn’t very good at learning this stuff yet. The symbols are unfamiliar, the sounds are unfamiliar, and it just keeps coming.
When you start to feel the pressure of not remembering, the first thing to do is slow down or stop lessons for a little bit, continue with the reviews, in the grand scheme of learning Japanese this won’t set you back at all. In addition to what I’m going to say below, reading, watching, listening, and then speaking Japanese should be the focus of learning the language, WK is just an assisting tool, don’t get too hung up on it.
Once you’ve got some of the stress out of the way, the next thing to do is have a proper process for learning kanji and really stick with it.
For me, the initial process I followed was:
- Mostly ignore the mnemonics given.
- Create a small story that went from the radicals to the meaning of the Kanji.
- Have a story element for every sound that any kanji can make, and add that to the story for the meaning. I had to keep a note of them because my memory is rubbish.
So… for a simple one: 情, feeling, has the soul and blue radicals, makes the sound じょう. The story would be about Joe (じょう) feeling blue. Early on these mnemonics were pretty outlandish with visuals / sounds to make them memorable, now it would be “Joe’s soul is feeling blue”. Then, on first recall after learning, ignore the fact that you can probably still remember it and go through your mnemonic to get the answer. Later, you look at the kanji, see soul & blue, oh that’s “Joe’s soul is feeling blue”.
Then you need a process for learning words. Some are nice and simple and just make the sound of the kanji and have an obvious meaning. For everything else, listen to the sound of the word, repeat it out loud - this is especially important for rendaku and small tsu replacements, saying it helps you get a feel for them and eventually you end up being surprised when a word doesn’t do it when you expect. If its meaning or sound is at all non obvious create a story linking the kanji to the meaning and the sounds in the same way you did for the kanji.
Another simple one: 幸い, “Simon, why are you always so happy?”, さいわい, happiness.
Once you have been doing this for a while there’s a later stage. Remembering kanji becomes easier, and you will be more interested in the differences between similar kanji and similar meanings, so you adapt your process accordingly.
At the same time, you’ve got a bunch of words and kanji tucked away so you can start to use them as well: “Ah, what was the onyomi for head? It’s in 叩頭 which is こうとう so it’s とう. In fact the mnemonic was something about head being next to some toes…” etc.
I have done a bit of kanji writing practice to help differentiate similar kanji, and it did work. Too slow and too much effort in the end to use all the time though.
HTH.