That textbooks, online tools, and other tutorials will only give you a very very basic sketch of your target language. Most of these resources make little to no effort to teach you the nuances of a word. Perhaps this is for a pedagogical reason, perhaps its a shortcoming of the tools, I'm not sure. To really understand a word, I try to look at as many examples as I can. Sometimes I like to ask a speaker of my target language "what is the first thing that comes to mind when you hear ___?"

Translations of words and phrases are just rough approximations. Querer does not mean to want. It means querer. No other language is equipped to describe exactly what meaning the word carries.

People are not as introspective about their language as you might like them to be. Sometimes I've gotten better answers asking Spanish learners than some Spanish speakers. This goes for everybody, myself included. I know how to use the word 'such' just fine, but if you asked me what it meant, I don't quite know what I would say.

Slang/informal language is the hardest part, and is very important when it comes to sounding natural in your target language. It's difficult to teach because the meanings are subtle and vary a lot by time/region, but I really wish language learning tools put more of an emphasis on this. My primary goal is not to write high school essays in Spanish. I'd like to actually speak it.

Listening and speaking are 1000 times harder than reading or understanding concepts in your head. It can be discouraging, having these great Spanish conversations in my head, then hearing a Spanish speaker on the train and understanding exactly none of what they're saying. Don't be discouraged be this, it's supposed to be the hardest part of a language.

Don't compare yourself to native speakers. It took them no skill at all to learn their language. You're learning a language without all that synaptic plasticity you have during your critical period. That's pretty badass when you think about it.