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!
This is exacerbated by the fact that in my 30s, the amount of alcohol needed for a hangover is quite low.
To be fair I can barely put a sentence together when sober either.