I learn any languages mostly alone, without any school or tutors (as it is insanely expensive and also quite inflexible). I learn it mostly online, starting wit apps like Rosetta Stone and Memrise as well as a grammar book, and once I leap over a beginner line, I keep reading Internet articles that attract my interest. If I hit a wall, I resort to an online community, including the Stack Exchange.
In this learning method, I can only improve my reading. I also chat with online friends sometimes so it improves my writing a bit (never on the same level as my reading). But listening and speaking, especially speaking, is very limited.
This happened on all languages I have ever learned. As a self-learner, reading is by far the easiest to acquire, while speaking is the most difficult.
Then when I travel to a country that speaks the language, I even cannot talk about the very rudimentary topic, such as "How many days do you stay in the current hotel?" or "Why did you drop out of college?". This keeps going even after I stayed in the country for months. (But these conversations are quite easy in written context.)
Arguably the strongest barrier is that unlike reading and writing, there is little or no resources online regarding listening and speaking that attract my interest. Another barrier is that I can't know if my understanding is correct, as basically everything online does not have the answer (such as the script for whatever they are saying).
If the language is relevant, it is Mandarin Chinese.