Don't give up. This is arguably life's most important skill in today's world. The most important thing is that you want this. You can talk your way out of or into almost anything. This skill can always be improved upon.
Never forget the most basic element is simply to convey what you are feeling with words. Which means you need to know words... not just familiarity, actually knowing exactly what those words mean.
it may sound crazy but reliving your past and thinking "what should I have said instead " helps . Read, find authors who you like because what you are really doing is seeing how others articulate their thoughts. it does not matter if you finish books just find books you love. Find people who you consider to be well spoken and study them.
Personally I realized when waiting tables that if I said "my pleasure " instead of "you're welcome" I would make more $$ in tips. As my income started to climb so too did my desire to be more well spoken.
do not over use the word literally