I usually chew a gum while playing because that just reduces my stress. Sometimes, I turn the volume low...really low...even mute and play like that because it's more relaxing ( even though it's a rhythm game and you should follow the rhythm XD )
If I get too nervous that my hands start shaking, I take a short break, drink some water, stretch my arms, take a deep breath and finally go back to playing.
If none of these work, I just flip a table and
"give up for the day" ._.
Also, try cheering yourself while playing, it helps a lot. I usually shout in my head " This is a piece of cake! " " I rock and nothing can stop me! " even if I get 100s and 50s all the way.
Just believe in yourself and don't let anything stop you.