I was once like this... I knew what I had to say but when I had to say it, I went blank, like a light switch flicked in my brain.
I'm better now thanks to more consistent writing and honestly, good movies.
You'll get better, slowly but steadily.... Write more.
The problem is. I could explain by writing well, but when it comes one-on-one talking for explanation, they don't understand my explanation. I have been like that for years. It seems it's not improving at all.
The only one who can understand me sometimes is my mother, and she's struggling to understand me. It's like we're playing a charade :').
You may benefit from seeing a therapist.