One technique I have found useful in language learning is to find some content, whether it be reading or listening, which is a bit beyond my comfort level in terms of difficulty, and practice with it for a period of time. For this to be effective I would say it should be for at least days, ideally weeks. Then I switch my schedule to something a bit easier and there the magic begins.
I’ve used this technique before, but I just remembered it recently because I had listened to a very difficult Japanese podcast for some time for a period of weeks, and when I had listened to all of the available episodes I switched to a different podcast which was a bit easier. The second podcast was in a field where I knew more vocabulary (science) and also was more conversational, whereas the first podcast was dry economics news packed with difficult terms.
In this case the second podcast seemed really easy to me, such that I was surprised at how much I understood even on first listening, without even looking up many words in the dictionary. I think happened is that my brain was so used to trying to listen so carefully to the first podcast, so when I switched to the second it was relatively a breeze. Though I never got really comfortable with the harder content, that harsh training was enough to help me improve my comprehension on easier content.
Surely, part of what is happening is simply a perceived comprehension difference – in other words, it may be that my comprehension of the easier material didn’t change that much but just seemed to be easier in comparison. However, I strongly feel that it’s more than just perception – I am actually understanding the easier material much better than before.
Regardless of how much the gain in understanding is perceptual vs. actual, the great thing about this way of practicing is that it really gives great satisfaction, and a feeling of improvement.
Depending on your Japanese level, you can select your “hard material” and “easy material” for appropriate difficulty levels. For example, you can watch a drama which contains a lot of quickly spoken, unclearly enunciated Japanese, and then follow that by a Anime series where the characters speak much slower with clear tone. Just be careful with doing this for reading practice, since jumping from content packed with Kanji to one with mostly Hiragana might not work too well, especially if your recognition of hiragana is slow (I have this problem sometimes). You can probably get good mileage of this strategy if you can jump between Japanese with advanced grammar and not-so-advanced, given the ratio of Kanji and Hiragana is roughly the same.
It’s interesting that a similar technique is done in some other domains, including sports. For example, an athlete can wear small weights on their body during the day, such that when they take them off for training they feel much stronger and quicker. I’ve heard boxers do this but not sure how common of a practice it is.
Also, I remember one time I was playing drums for quite a while with earplugs in, and then took them out and hit a single cymbal with my drum stick. Not only did it sound loud, but I heard extra nuances of the sound I had never heard before because my ears were so used to straining to hear a muffled sound.
See if you can try to incorporate this two-step process into your daily language learning studies.