I’m not a polyglot, but I’m fluent in English and now lower intermediate / conversational enough in Japanese within 2.5 months.
Get the idea that “language learning is hard” out of your mind first. Get all of it off. When people rant about how German grammar is hard or Kanji is hard, ignore them. Just focus on doing and studying.
What I found is consistent, everyday learning is MUCH more effective than weekly class learning. To me, studying everyday consistently even yields much better result than if I skip a week, or even just one or two days. When you have gaps between days, you forget things you learn between them. Studying everyday is more intense, review-heavy and provides more incentive because you progress faster.
Even if you’re not studying textbook, keep immersing yourself in that language. Try to read fairytales or product packaging. Find the vocabs you don’t know in dictionaries and add them to your SRS deck.
Use SRS / Anki to remember vocabularies. It works.
Try to memorize with stories or comparisons. With Kanji especially, I found that remembering radicals, comparing with similar kanjis, and creating stories also really helped.
Utilize your time on the fly, especially on commute. I can finish my deck of 250 vocabs during commute alone.
When you’ve reached an intermediate / upper intermediate level, move on to literature and read shitloads of book in that language. I personally reached my English fluency through this- I don’t even know how many tenses there are in English. All I know is some sentences feel “right” and some sentences feel “odd” to me.
Set a higher studying / daily goal than you think you can do. Sometimes you can do better than you yourself estimate you are.