it takes practice. you need to switch your thoughts...
here's a formula that works every time:
thoughts + feelings = emotions.
if you are feeling jittery, you can either think about something exciting or something fearful to get a different emotion.

for instance... if you are anxious, you are probably thinking of something fearful and feeling jittery.
if you feel excited, you are thinking about something awesome and feeling the jittery sensation. its just what we focus on.