When you start learning a language everything is unknown. You take in vocabulary and grammar and make fast progress in making those new, unknown words become old, known words. The speed of progress becomes normal.
What most language learners aren’t prepared for is the plateau at the intermediate level, where there is less clear meaning. As a beginner, you studied the difference between ‘apple’ and ‘orange’; as an intermediate learner you study the difference between ‘freedom’ and ‘liberty’.
What can you do to feel more confident in your language use and studies?
Love the lack of clear difference
Enjoy the language like a native
The lack of clear difference means you have made so much progress already. You are ready to learn the different nuances (small differences in meaning) between similar words, try longer sentences and use more complex grammar structures.
This means you don’t have use textbooks as often as before. If you have a grammar reference book, a dictionary and a notebook you can write down the interesting language usage you see and hear in the things you read, watch and listen to. Remember, native-speaking children do not understand every word of all they hear from the television. They guess and they are used to making mistakes. Relax and enjoy yourself.