I just purchased a chuck it tennis ball that comes with the throwing handle for my dog. Just about 15 mins after I bought it and tried it out with the dog, she loved it. The throwing handle is a little hard to adjust to for me so I was throwing it quick close. I managed to throw it pretty hard and the tennis ball hit my dog right in the eye! My natural instinct was to coddle my dog and apologize. I felt so sad that I hurt her, she was squinting. Now she's completely afraid I'm going to hurt her, especially when carrying the chuck it. How do I regain her trust and get her to love the toy again? She definitely thinks it's going to hurt her every time I pick it up. Though it's only been a few hours since I accidentally hit her.
I agree with Mr Kennedy. Another thing to reassure you:
Dogs always live in the moment. Their brain is not able to look in the past and feel the moment as we humans can do. That said, new research has shown that they are one of the only species who can read human emotions.
The only way your dog will "remember" that you've hurt her whilst playing is that if you feel that bad moment and remind her when she reads your emotions. That will trigger the dog back into that moment.
Mr Kennedy is right about the training, which is behaviour replacement and proper dog training. However, it won't be that successful if you feel guilty and keep the dog in that moment.
Start slowly. Reintroduce the toy. Does she cower from it? If so, put some smell or flavor on it. Let her sniff it/lick it. Then put it away. Repeat until she stops cowering. Put the toy(s) out of site or in a bucket when it is no longer play time.
Next, if she is no longer cowering from it, non-chalantly use it to groom and pet her (I do this to get my dogs okay with the nail clipper). That will get her smell on it and build an association of good feelings.
Do likewise with the tennis ball if she cowers from the presence of the tennis ball.
In the meantime, try fetch with a stick. Get some gloves if you don't want to dirty your hands picking up the stick or toy. Rebuild her fetch=FUN! habit. Once she's okay with the long handle ball thrower, back at it.
Do note if you go the stick route if she cowers from the sight of you holding up a long object.
Be reassuring with your voice and with petting/belly rubs/ear scritches for good behavior and following commands. Contrast this reward voice with your command voice.
Most of all - have fun :D