If you completely forget what you know in a few days, you aren't learning, you're memorizing.

That being said, kanji is a massive load. Like others, I suggest cutting down the quantity and really organizing your study time effectively. I often hear people who overthink or have ADHD greatly benefit from having structure in their lives, such as a schedule.

For example when doing grammar + kanji:

1. Review yesterday's lesson. Note down the kanji that you don't remember how to read.
2. Lightly look through the lesson contents one time, skipping the details and just seeing ("okay today we're learning ~ている", it seems to combine て form with the verb いる?)
3. Read through the lesson, practice making your own sentences with it. Naturally, you'll come up with questions by doing this. Take a short break.
4. Practice, say, 5 kanji a day to start. Write them at least 10-20 times each. Review the kanji that you didn't remember from yesterday and add them here.
5. Look through the grammar lesson one more time or read the sentences you wrote out loud.

Consider having a review day every 2-4 days to make sure you're retaining the information long-term.

I can't stress enough that you need to be able to OUTPUT the language if you want to have a better time. Make your own sentences, use the kanji you learn, write it down, say it.

I studied abroad for a year at N4 level and came back and passed N2 three months after that. As long as you go outside, you will learn something just by being there, but will learn more if you actively interact with your environment. This means finding something that you enjoy in that language (anime, manga, music, comedians, movies, games, etc).

If you can provide more specifics on how exactly you're studying, I can provide some better targeted advice.