Fear can be useful as Andy said. It's very dangerous to ignore fear but it can also useful to know how to bypass it where necessary. Being an emotional response, fear tends to respond based on long-term patterns of behaviour and occasionally it's misplaced.
Just simply for calming your mind down it might be best to find some kind of general relaxation technique that works for you. If you let your body relax then you mind will find it easier to relax too, which will leave it more effective.
One technique that works for me is to forget about the movement and turn all my attention to my internal sensations, such as breathing or heart rate. Becoming consciously aware of them can help you regain control of them if they've become highly active as a result of the stressful situation, and also acts to clear your mind of all the confused thoughts and feelings about the problem movement.
If needed, I shift my focus onto each individual muscle in turn and make sure it's relaxed before moving onto the next. Once I'm sure I'm relaxed then I can turn my attention back to the obstacle and look at it afresh.
That sort of thing you can do fairly quickly once you get used to it, maybe only taking a second or two to reset your mind. If there's still a problem, then you need to start finding ways to make it easier. You can become more familiar with the technique by practising on other obstacles, try other movements on this obstacle to get more familiar with the obstacle itself, or just keep training to learn about and improve your ability.
Practising in a soft environment is a good technique to use sometimes, but remember as well that you also need to practise dealing with situations where failure has negative consequences, where that fear will still exist.
