Jitters and nervousness come from your brain worrying about something in the future—which could even be the immediate future.
In order to calm the brain, you have to bring it back to the present where it can actually do something to impact change.
In Psycho-Cybernetics, Maltz says to think about what is here and now that you can do something about.
So, if you are worried about an upcoming meeting, stop stressing about the outcome and instead focus on leveling your breathing, and doing whatever you need to IN THE MOMENT to feel prepared and confident.
This will relieve the jitters. And remember, to do the breathing exercise I taught earlier in a post to let go of the negative in the brain.