Just when you've nicely settled into your target language and you are more or less comfortable with its quirks and challenges, the next monster arises: the dreaded Intermediate Plateau.
The Intermediate Plateau is a phase mainly characterised by disorientation: things are no longer as clear as when you had a beginner-level course that you could simply follow in order to be sure to make progress. Even if you found an intermediate-level course for your target language, it is uncertain whether this course will catapult you forward in the way that you are used to. You may be trying different things and none of them seem to have an impact.
There are two reasons for this:
1. Diminishing returns. As a beginner, each 100 words that you learn mean a big step forward. Going from knowing 50 words to knowing 150 words? A game-changer. Going from knowing 500 words to knowing 600 words? That's still a 20% improvement and you'll encounter at least a few of the new words every time you practice the language. At the intermediate level, each 100 words may just be a 5% improvement, and you may never encounter most of these words again if you collected them from an intermediate-level course or a frequency list rather than from a topic that you're regularly engaging with, so you miss out on the "Yay! I couldn't say that last week but this week I can!" moments.
2. The "hammock". It is very easy to find the language-learning equivalent of a hammock and stay there. Have a call with a tutor, talk about the weather and what you did yesterday, listen to a few target-language songs you like, call it a week. If you keep doing this, you will very quickly have learned almost all the words you'll ever need for these types of conversations and these songs, and your ability won't grow.
How to overcome the plateau
You have to challenge yourself. Maybe even literally, through one of these challenges. But the key point is to consume materials and have conversations (including written, on forums) where you regularly notice that you're missing vocabulary... and to stay on that topic until you notice that you aren't regularly missing vocabulary anymore.
Fluency is never more than topic fluency: first you become fluent in talking about yourself, then about everyday topics, then maybe an area of interest, but you'll never be fluent in all possible topics. I'm fully fluent in English and I still cannot talk even high school level chemistry in English. By contrast, I find it challenging to talk about economics in my native German, because economics is a topic I've only studied in English. This is the secret to overcoming the plateau: pick one topic, become fluent in it, pick another topic, become fluent in it, pick a third topic, become fluent in it... until there are no more topics that are useful/interesting to you at this time. (You're allowed to come back to it later.) With each topic, you are stretching yourself, you are learning a ton of new vocabulary and expressions, and you are upgrading your knowledge from intermediate to advanced.
Note that in order to pass the advanced levels of official exams, you are required to have politics / social issues among your fluent topics, i.e. being able to understand and to comment on texts covering e.g. climate change, pollution, poverty, aging populations, the prospects and perils of technology, cultural differences between your country and the target country, and so on.
I have reached an advanced level in many languages already. Here are a few examples of how I did so:
English: I joined a "Global Affairs" forum where English native speakers were commenting on current events. Not shitposting; most of them were university graduates writing multi-paragraph commentary that could have been published as a newspaper editorial. It did wonders for my vocabulary and essay skills, especially once I started writing there myself.
Chinese, first time: The first time I got stuck in Chinese, I solved it by getting sucked down a rabbit hole. Rabbit holes are language learners' friends, as long as they are in the target language. In this case, the rabbit hole was Romance of the Three Kingdoms (ROTK), one of the four greatest classics of Chinese literature. I started by watching 1995 TV series on Youtube with English subtitles (80+ episodes). Then I wanted to watch the just-released 2010 remake (another 80+ episodes), and this series was only available with Chinese subtitles, so I got better at reading characters. Then something weird happened in one of the episodes and I wanted to understand it, so I googled discussions of that episode, which were, as you guessed it, in Chinese. That day I learned how to quickly skim Chinese text in order to find the part that I'm interested in, and I spent several hours reading Chinese just that one day, going from link to link, deeper and deeper down that rabbit hole. It wasn't the only time that happened.
Chinese, second time: After that, when I got stuck again (at a level suitable to watching ROTK but not truly advanced), Richard Simcott helped me out. He convinced me to join university classes that forced me to regularly do Powerpoint presentations in Chinese, debate social issues, even talk about linguistics in Chinese and write a linguistics essay in Chinese! All of these forced me to develop a vocabulary beyond what's necessary to talk about everyday life and my Chinese improved by leaps and bounds. (For those interested: the classes are offered by Dalarna University online and they are free for EU citizens.)
Serbo-Croatian: After completing my beginner-level textbook and feeling stuck, I bought the first level of a series of easy readers by Ana Bilić. When I finished reading that, I bought the second level, then the third, the fourth... laddering up that way, at the end I was able to start reading books for native speakers. The vocabulary I learned this way has also helped my listening comprehension of TV series.
Modern Greek: I was never a typical beginner in Greek because I more or less started learning it by reading news articles (when you're really passionately curious about a topic, and you use the Reading Tool or an equivalent browser plugin, anything is possible). However, when I decided to develop my speaking ability, I quickly hit a plateau where I could talk about my everyday life but I wasn't developing further. This is because I am very good at rephrasing what I want to say in simpler words, which is essential for my "quick and dirty conversational ability" plan, but it can be a problem at the intermediate level, where I might say "What did the Prime Minister say last night?" rather than "What did the Prime Minister announce last night?" and so on. The solution has been to force myself not to chatter away but to deliberately pause and try to come up with the most suitable word, which I probably know very well passively but not actively.
I hope these ideas are helpful to you! Good luck with your studies!