I have an extensive vocabulary but I can’t use it

I have an extensive vocabulary but I can’t use it I would say that I have a good vocabulary in general, but for conversations I can’t use it because I stutter. My mind would know exactly what I’m going to say, but when I get to the word, I substitute it with a simpler filler word, thus making my sentence or argument less profound. There has been so many times where I would be really well versed on a topic, but I have to explain it like I barely know it or in a way it doesn’t make sense because I can’t use “big words”.