Whenever a reaction is larger than the person feels that it should be....the reaction is usually being magnified by the person's past experiences.
While it is normal to feel regret and sadness and guilt over actions that we find unacceptable, a reaction that goes on for a long time, like this one, is usually the result of emotional "unfinished business" from the past.
In your case you were abused, and the action that you took on your dog is most likely in your own mind, abusive. That action would result in what is called an emotional double bind. You were trying to help your dog, wanted to do a good thing, but your reaction was larger and bigger and more severe than you would have liked. This in turn caused you to feel guilt and shame, and because those feelings are unresolved from your past the feelings remain, strongly.
What to do: Working through this is not only possible but very good for you to do emotionally. A great place to start is the resource: Adult Children of Abusive Parents by Steven Farmer. It is inexpensive on amazon.com and is simply excellent. It will help you work through your own past and assist you in getting past these powerful feelings. The guilt about your dog will go away as you do so and you will feel better overall. Sometimes like iron sharpens iron we need a little boost to help get past these issues The resource I mentioned will do so, very well. Steven