Racing is a mental game, so much of your performance and potential relies on the right mind set. Its easy to psych yourself out by focussing on the wrong things.
Its important, before your race or qualifying, to not get too wound up. Find something to ground you, to keep you a bit more relaxed. That might be telling jokes with your crew or listening to a particular piece of music, something which allows you to be focused but not over-anxious.