How do I know what I need to learn?

To clarify, I'm not asking what I need to improve at. I'm asking how I can find resources to help myself improve. First off, how can I identify where I'm weak and need work? Then, how can I find the right materials to target that issue? Say I'm trying to learn about middle-game play. How would I go about finding the right materials to learn from? How can I find the right books/sites/videos for my skill level? Thank you.

"how can I identify where I'm weak and need work?"
Analyse games you lost.
"How would I go about finding the right materials to learn from? "
The tactics puzzles on this site are good.
"How can I find the right books/sites/videos for my skill level?"
A good book is good for any skill level: everybody learns something from it.
Judge books by the strength of their author. Learn from the best. If a book by a mediocre author were good then how come its author did not get better by writing it?
Books by Lasker, Capablanca, Alekhine, Nimzovich, Euwe, Botvinnik, Bronstein, Fischer, Kasparov, Kramnik are recommended.

Some books that you can use include tests that are examples from the material in the book that can help you see if you know the material or not. Seeing your skill level, I'm going to recommend the book 100 Endgames You Must Know, by Jesus de La Villa. It includes vital information on endgames and tests if you know the material by questions. Even though it may be a little advanced, I still think that it improves how you do in endgames. Even though I'm only a 1700 and this is a book on only endgames, I hope this helps!(I'm sorry that I don't have any middle games for you)

I would suggest one of these interactive books, they might help you pinpoint what exactly you need or want to improve
www.chessable.com/the-fundamentals-build-up-your-chess-1/course/19145/ or www.chessable.com/tactics-time-1/course/21108/ are worth the time

My advise (and how i train myself) is, play classical games. Afterwards analyse it, first without engine, then with it and see what you did wrong. Then the most important part is, why did you make a certain mistake? Did you miscalculate -> practice tactics. Did you make a positional mistake -> learn about positional structures ... I would also recommend not to train too much at once, because your brain needs to adjust to new patterns you want to learn.

I would also recommend to play only one opening as white, and only one opening as black for each of whites openings. The reason is, you will have to deal with less different structures, so you can improve on those structures faster.

If you want a book: Khmelnitzki - Chess Exam

If you want a blog: http://www.qualitychess.co.uk/blog/category/jacob-aagaards-training-tips Start with the oldest and read it "from front to cover". There is some material you will not need, as always.

you could also try to get a coach if you are 1900 or under. Coaches are good at analyzing games played at tournaments and showing you what you need to learn.

I would suggest that you read "chess for tigers" by Simon Webb. There this will be answered and also this is one of the best chess books out there. It does not teach you any ideas/tactics but has more the focus on how to play good as a human and some valuable tips and tricks how to play.

This following idea is a life skill. It works for chess too.

Having confidence is one thing and proving your strengths is another.
Test you skills and discover your strengths. The rest can be considered areas to improve on.
Then decide what interests you and what doesn't.
Study the things that interest you.
If you have no idea, read through a glossary about terms and discover what you don't know or don't use and then search a wiki on the subject that interests you the most.