As hard as it was this is the best thing for all of you.

You did the right thing. And now you know to see how the other animals will react to a new pet before committing to anything. Lesson learned. It's tough with cats and dogs though. You can't exactly take the cat to the shelter to see how it interacts with the dog. But maybe you could find a dog that has lived with cats before so you know its a good match for your household.