iggy:
It sucks when your dog goes after your cat. I had a foster puppy who, as soon as his cast came off his leg, started going after my cat relentlessly. Previous to that I had been thinking of keeping him, but after he started pulling my cat down off her perch by her tail I changed my mind. What kind of dog is it? I'm worried that you described her as aggressive. If you don't nip that behavior in the bud it can become dangerous, and at that point you don't have a pet, you have a liability. Now, I don't know everything about the situation, but it seems that you may not be the right owner for this dog, and that's okay. You shouldn't feel forced to keep her because you feel bad that she's been bounced around so much. From what you've described, she seems to need an experienced owner that will have her as the only animal in the house and will work with her on a daily basis. Right now, the situation isn't fair to any of you. There is no shame in sending her back to the rescue. You all deserve to be happy and right now, none of you are. If you choose to keep her and work with her make sure you hire a good trainer that uses positive reinforcement. Negative reinforcement can only make a dog with stubborn or aggressive traits even worse. I wish you the best in this sticky situation and hope everything turns out great. 
punkinhead:
@iggy - It is a puggle, the crossbreed pup of a pug and a beagle. She only weighs 13 pounds or so, and she is really only aggressive with the cat and occasionally other dogs we meet on our walks. But when she gets riled she goes all out.  Thanks for best wishes. The dog really is here to stay - my wife would no more send the dog back to rescue than she would forgive Michael Vic.  See, she is mostly happy because when she is home the dog (her name is Bobbi, by the way) follows her around and just sits with her when she is home. But if my wife picks the cat up, or even pets her, Bobbi rushes over ready to defend her territory.
grumpyemt:
I feel for you. I am lucky my cat and dog get along. It is really hard to take a rescue animal back. I have had to do that once with a rescue cat 5 years ago. I picked it up after my other cat died. The rescue cat was good for the first few days. Then it stopped using the litter box and would pee and poop on the heater vents. I took it to the vet to get checked for any illness, I tried 4 different type of litter boxes, every litter under the sun, and after 6 months I finally took the cat back to the rescue with copies of the vet paper work showing a healthy cat that could not use a litter box willingly no matter how much training. That was hard as hell
punkinhead:
I bet that was rough, @grumpyemt . I don't think I can do it. And I really do believe that in a year or so (that would be three hundred and sixty five fucking days), if we keep up with her training she will probably be fine. Le sigh. You should see her and my wife - the missus sleeps holding it like a teddy bear. If I said the dog has to go I would feel like a combination of Scrooge, Cruella de Vil, and that jagoff that wouldn't give Oliver Twist any more oatmeal.
grumpyemt:
I totally understand. I felt horrible taking the cat back but spending anothe $200 for duct cleaning to get rid of the cat urine smell from where it would pee in the heating and a/c floor vents I only felt midly awful