OK. This is a situation where her age, the coughing, and her breathing harder when she's not coughing have me concerned. If she was around a lot of dogs who had coughing or upper respiratory signs, then a case of kennel cough could be suspect. But given the other dogs she's around (and I'm assuming that they aren't coughing or acting ill), it doesn't seem like she has a lot of exposure. The coughing could be due to primary lung or heart issues. My best advise is to get her to your vet as soon as possible for them to evaluate her. I know you have a problem getting her into the car, but maybe a friend or neighbor can help with that. At the clinic, the staff there can definitely help. At home, all you can do there is to let her rest and to see if starts to breath better. That isn't much. Getting her to the vet will allow them to see what more can be done to help her get through this.

I'm hoping for the best for this girl.

I'm sorry that you have a hard decision to make. Did they diagnose her with heart disease?