Well, MXer, the good news is that you've obviously got the technique and skill to go fast. That's the hard part.
The raceday jitters or lack of "flow" isn't anything unusual. People get it during public speaking, before interviews, first dates, music recitals...just about all areas of life. It's a very subtle performance anxiety or self doubt. Pros get it after recovering from injuries. Other athletes develop little rituals before stepping up to the plate or leaving the dressing room. You'll overcome this in your own way and then wonder what the big deal was in the first place. It might require entering a race in a different organization, sort of a race with no pressure or consequences, or if you have friends/family present at the race, maybe they could take a walk during your heat so you don't feel a sense of being observed. I'm not worried about you; you sound like a human being to me!