Well, drat, I'm so sorry to hear this.
When my girl's murmur advanced rapidly, I put her on low dose doxycycline and it stopped most all progression. She always had a weird infectious component, that we never figured out (lyme heavily suspected), and I do believe that there was a lyme or bartonella component to her heart disease. It was a low level murmur for a very long time, then progressed, and one of the tendons in her heart broke (luckily, it was a minor tendon).
She was put on enalapril, we didn't need the lasix, she luckily never went into full CHF. She did not die of heart disease, but of a brain/spinal lesion (again, thinking of that infection
Anyway, in addition to the doxy, I stayed away from things like Hawthorn (commonly recommended), because there is a balance between trying to make the heart beat too strong and stressing the structure of the heart. I did add hyaluronic acid to keep the parts of the heart supple (not glucosamine/chondroitin, but HA). Fish oil is important, to keep the muscles from atrophying. I guess that would include the heart muscle?
I hope that this link will work, I thought this was very helpful: