I suspect that after, as well as, the rational learning part of language learning when you pick up vocab and grammar there is a need to learn common phrases and words that are commonly used together and to make these part of how you think. I suspect hat this only comes with practice, perhaps both hearing others and reproducing them yourself. When your brain is thoroughly used to the combinations, phrases, collocations, then they come more easily. Sorry, not a wise nor easy answer. It may be that you have already done a lot of the "hard" part. IT may be that the stage that holds your next advance is not so difficult but a litt;e time-consuming. . I do admire how far you have gone already :) . I can read many English papers and and I understand the English they say, but I it takes me time to form my sentences with correct grammar. It's difficult for me to say a sentence quickly and not use many simple words. Depends also on your aim. Many people speak with complicated language but do not say things any better than some who use simple expressions. :) Aim for fluency and having the choice, but sometimes simple is good.

That’s completely understandable! It can be helped by further speaking and writing practice. Book some more lessons and get practicing :)

You can try to talk with other people in English

This is not uncommon. Second languages are challenging. Just keep talking. I sometimes will watch a YouTube video etc and stop the video every few moments and try and repeat back exactly what they just said to mimic the speed, expressions, etc. You will get there if you just keep talking. It's a life long journey.