A coupla things:

1. The more you do, the less nervous you will be. Once you realise that nothing is really at stake of any import, you will be fine.

2. Think of each race as training or preparation for the next race. It helps to have an "A" race to fully appreciate this sort of mindset. Everything else is just preparation for those one or two important races. Otherwise I find I get into a mindset of having to do well in every race: the most important race is the next one. That attitude will burn you out. A lot of "burnout" that I see is more psychological than physical.