My cat has become completely apathetic after putting a buster collar on it. What should I do?

This is unfortunately quite a common problem. Most cats get used to the buster collar after a while.

If your cat is within close vicinity it is OK to remove the collar for shorter periods of time. You must put the collar back on as soon as you do not have your full attention on your cat. The collar must also always be on when your cat is alone and at night when you are asleep. Try to distract your cat by playing with it and cuddling with it. Offer your cat its favourite food and help feeding it. Your cat will also appreciate you cleaning its face from time to time. 

If you are feeling hopeless you can try using a cat body suit if the wound is situated on the abdomen or back.

It can be wise to try to get your cat used to a buster collar before it needs it. Try luring your cat with a treat and get it to stick its head through the collar. Leave the collar on for a little while and repeat the process until it gets used to wearing the collar.