How to Learn a Foreign Language by Paul Pimsleur is a short book that has a lot of gems about language learning.
My tips as someone who has learnt a foreign language to fluency after becoming an adult:
Focus on sounds before the written language. Pimsleur's courses are good for this, but also just watching TV shows etc in the language you want to learn. The flow, rhythm, any extra sounds etc are all important. Mess around and try copying what people say when you hear it. Ham up the accent.
Try to learn everything in a context. "Where is the money?" as a line in a tense movie thriller is far more memorable than reading it in a textbook. The brain loves little connections. It's not like a hard drive. Remembering more is often easier than remembering less.
Don't feel you have to learn any particular thing right now. If some word/phrase/grammar form is hurting you just put it down and come back another time. It might be months down the line. You will have probably picked up more knowledge and more examples in those intervening months so learning it will be much simpler.
Use materials that are intended for natives. Books for learners are more likely to have mistakes or have very unnatural phrases in them. Sometimes they swap out the more common phrase for a less common one because "We haven't learnt that yet". Have a healthy suspicion of textbooks.
Learn how to use a dictionary. Look up words you already know to help you get the measure of a dictionary. Look words up in one and then cross check in another. Always be aware of the fact that a certain idea may be very common in one language but uncommon in another, thus the words may match in meaning but be wildly different in frequency. Context and materials for natives helps a lot here, if you're reading a Spanish magazine it will contain a good cross-section of common Spanish words.
Have some way of reviewing words. I recommend a flashcard app like Anki[1]. While useful, try not to spend too much time reviewing. Flashcards are just something to remind of what you learnt last week, not to learn new words. If you want new words, go back to native materials.
[1] www.ankisrs.net