Two things in addition to whats already been said..
1. Follow up the tennis ball drills with a machine sending short of length balls (bouncing between your waist and chin)...stay relaxed, still head..and focus on getting behind the ball and playing it VERY VERY late, right under your eyes, with soft hands..you'll soon realise that you have plenty of time to react to any "nasty" stuff
2. This is slightly unconventional, but taking boxing / Muay Thai classes can help. You can't afford to close your eyes when someone throws a punch - you have to keep your eye on it and react accordingly. Great workout, and I think it can help overcome the fear of the ball. But as i said, this is a bit weird and not everyone's cup of tea.
But keep at it mate - you'll be back in no time.