Does the doubt come from lack of practice or poor technique?
Fear of losing the point or 'looking bad'?
Find the source of your doubt and you may be on the path to eliminating it.
For me it was mostly lack of practice which led to lack of confidence in the second delivery. Look to generate plenty of spin. This should help with both consistency and accuracy.
In the end remember this- only by focusing soley on what
you are doing can you control how
you are doing.
Good luck and don't forget to have fun!