I was similar to what you are experiencing. When I was nervous or anticipatory, like knowing I was getting called on in class, public speaking, phone calls, ordering at a restaurant, my speaking was not good. I was filled with blocks and could not get out certain words. Off the cuff spontaneous speaking was ok, probably a 7/10 or maybe better, we are our own worst critics. Certain words I could not say but worked around them. As I got older, my speaking got better, I don’t know which came first, being less nervous and speaking better or speaking better which made me less nervous. I think it was a positive feedback loop, I was more comfortable with myself, less nervous, so I spoke better and speaking better made me less nervous and more comfortable with myself. Based on your info, your experience sounds similar to mine.