That is the 'charm' of OTB chess. We all get's nervous. Even the very best. And finding personal ways to deal with it (it cannot be prevented) may be as valuable as trying to improve your chess understanding and skills.
Try to search some information on sports psychology. Personally I have benefits of breathing exercises and having something warm with me as I tend to freeze when nervous.
I have a question specifically related to over the board tournament play. Even in low-stakes tournament games, I find myself extremely nervous (visibly shaking) in the waiting time between my move and my opponent’s. I don’t feel this during my move or while they move, just the time in between. I find it incredibly hard to calculate and concentrate during this time. Why might this be and how can I calm myself?