In reply to KellyKettle: as mentioned earlier Dave Mc's book covers this in an effective way.
It is however a pretty standard sports psychology problem , and i guess you should be able to find some excercises to deal with this. However i am not up to date with sports psychology books and can't recommend one. Personally a long time ago I actually had sessions with a sports psychologist as I was getting similar competition stress. This will be pricey tho'.
Some comments - avoiding crowds is poor advice as you are dodging the issue, and if you fix this you'll get a strategy for fixing other problems fear of run outs etc., But also simply going to walls and hoping for a sudden change and to get magically use to it is a difficult and not especially effective method if you don't have some other excercises or whatever to help you