I noe this is about a month late with this, but hopefully it will help you out.
For one thing your "jitters" are more or less your bodies way of prepairing itself for either new experience, and or simply getting ready by pumping adrinaline into your blood. At least that is how I have been explained to. If you relize this, as in the moment you feel this sensation, then tell yourself this is what is happening hopefully you will be able to use this rather than fight against it. The fear you think you are having is nothing more than not knowing what your jitters are and then more or less sikes you out. This will pass, win or lose in your fights. You will still get this feeling I believe, but you can acknowledge this and be able to deal with it more to your advantage. Further more, with experience you gain more confidence. Not selling anything, but here is something I found that you could look up for yourself. American Kenpo Grandmaster Edmund Parker's Infinite Insights into Kenpo series, where the first of the series deals with mental conditioning. Another thing I believe may help you is a book a personally own The Tao of Jeet Kune Do. In this book Bruce Lee for the most part explains, there is no fight. In other words you have to be as thou nothing critical is happening. I can not explain to you how to go about doing this other than try to practice shadow boxing and standing in one spot, pretend that someone is comming up behind you, or where ever you will, and fight back. Use moves you would actually use, nothing unrealistic, simple moves should be fine, such as a punch, kick or a throw. Also try meditation, where you start by breathing correctly. I do not recall the correct count, but you breath in thur the nose and then out thur the mouth. Here is the trickey part, you use your stomach rather than your chest. Ever notice how say a reptile breaths? If you watch, its the stomach that moves, not it's chest. Once you get this down, start focusing on one thing at first such as an object. Keep this picture in your mind and relax. Also do this in a comfortable position. Standing sitting does not really matter, just that you are able to consentrait. I hope this helps out.