Talking is a skill like any other.
So, a comparison.
You know how you can picture a car in your head but you can't make draw a car unless you've spent time practicing as an artist? That is because connecting our mental representation of "a car" and making our hand draw a car is a skill that requires a lot of training. This is why artists practice drawing still lives, looking at an orange and copying it down to paper with a pencil trains your brain on how to translate the thing that is visually inside your head to what your hand needs to do to re-create it.
Well, the same for mental dialogue. Going from "idea in head" to "words coming out of mouth" is a skill that takes practice.
There are ways to practice this! Toastmasters is a famous group whose goal is improving this skill, but there are many other routes. I had good luck with table top RPGs, I spent years playing a character who is supposed to be charismatic, and accordingly I had lots of chances to practices being outgoing!
We are good at the things we practice doing!