I want to save a dog
Why do you want to save this dog?
I'm already feeding it even though I don't want to
Its great that you are going out of your way to help this dog. While it is definitely not easy to do this, you are without a doubt helping this dog out big time.
I stopped visiting the dog, I've made the decision to stop seeing it.
Well if the dog is no longer around, then you shouldn't feel compelled to take responsibility for it.  However, if you see this dog again, it might be best for you to take it to a shelter, and hopefully they can find its owner, or a new home for it.