One of the most frustrating things in studying Japanese is coming across a kanji you swear you should know but still don’t.
Usually what will happen is that I’m reading something in Japanese, be it manga, article, essay, or book, and I’ll hit a particular word that I don’t know the meaning of, but still feel like I recognize it. Then it hits me that I’d seen this word previously, and I had been in almost the exact same situation, where I’d be looking at something and finding the kanji, this time determined to commit it to memory. Except I didn’t which is why I’m in that current situation in the first place. Ever break a promise to yourself and then forget that you did? It’s kind of like that, only I don’t ruin my friendship with me forever as a result.
A couple of recent examples include:
基礎 Kiso, meaning “basis.”
至る Itaru, meaning “to reach.”
Maybe if I just complain about Japanese enough, I’ll learn it.
Jokin aside, the real culprit is obviously under-use, and if only I’d keep up my studies more consistently this sort of thing wouldn’t happen. I’m reading quite a bit of Japanese lately so hopefully more of it will be able to stick, at least reading-wise. Spoken Japanese is another matter entirely, and I can feel myself not developing in that regard as much as I should (and possibly even regressing a good deal). I have to reassert my conviction to learn, as I have every reason to do so.
OGIUE MANIAX 
If it helps, there are probably native speakers who experience the same phenomenon at some point in their studies. Nobody knows *everything*.
I once had to give up playing word games (Scrabble, etc) with a significant other ten years my junior. After a time, it was just too painfully obvious who had read more and who had been raised by two teachers and who hadn’t; My vocabulary was so much larger and expressive mainly from greater life experience, but unfortunately my S.O. was a sore loser who couldn’t, or wouldn’t see that.
The coolest thing about learning a foreign language (especially more than one) is how much more you learn about your *own* language in the process, and how languages work in general.
So far I’ve stuck to learning European languages. Kudos to you for picking Japanese and sticking to it. I find Asian languages very formidable and intimidating.
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