You are NOT a terrible dog owner. Please don't even think that way.
You are a realist that is doing the best she can taking care of a loved one while maintaining the best quality of life for Heidi that you can provide.
I made similar decisions for my old girl Maddie before I had to PTS. She also had HD and arthritis and had a long slow decline.
Linus van Pelt Haus Juris CGC. My Sweet Babboo, born March 17, 2014.
Madeline Ronan Heart (Maddie) Dec 1997 -- November 22, 2013 Rescue GSD, Sable