I just made a post on a very similar topic, before I saw this one.
Heat844 wrote:
I know what I want to say, but have a hard time getting it out. Its like my brain is working faster than my mouth.
This describes me very well.
I think there must be some way to practice this. Maybe when you are alone (by "you" I also mean "me") you can work on just speaking ideas out loud as they come to mind. I bet after doing this for a while the skill for finding words will improve. But you would probably have to somehow reflect on how well you are choosing the words, so that the practice becomes deliberate and not just mindless repetition.