I assume that you’re pretty well into your studies at this point, so you’re probably aware that language learning can be split into two categories: input (listening, reading) and output (speaking, writing). While both input and output are related to a degree, you can treat them as separate skills and, consequently, should be trained separately.
When you learn how to listen or read Japanese, you’re training your brain’s recognition–the ability to understand what you see/hear. However, to speak or write, you need to train your brain’s recall–the ability to fetch that very same information. Your brain may have thousands of words and hundreds of grammar points stored inside; but it can only use the data to identify language, not construct it.
The only real way to improve your output in any language, not just Japanese, is by actually doing output practice. If you want to get better at speaking, you must speak regularly. This can be done in multiple ways, including, but not limited to:
- 1-on-1 conversation lessons with Japanese teachers
- Shadowing (repeating sentences spoken by native speakers)
- Self-conversations (assuming the role of two characters in an imaginary conversation, and playing it out)
- Speaking with other native speakers in online games and forums (places like VRChat)
You freeze up because your brain is thinking not only about the topic at hand, but also what word to use, how the sentence should be structured, etc. Natives don’t think about these other points because it’s something they’ve learned to do naturally through practice and reliance on common collocations/phrases. This is something you can only really train through methods like the ones I’ve outlined above.
Truth be told, I’m in the same boat as you; my recognition skills far outpace my recall skills, and I want to improve my ability to speak Japanese, though my current goal is to pass the JLPT this July, so it’s not something I’m focusing on at the moment. Still, I can only speak Japanese as “fluently” as I can right now because of everything I just mentioned, so hopefully you can get some mileage out of the methods I’ve recommended, too. Good luck with your studies, fam.