As an "older cat" --meaning not a kitten--her likelihood of being adopted is low, especially at kill shelters. You probably saved her life. Now that you are feeling like maybe you aren't doing enough, just redouble your efforts to feed her, keep fresh water change the box and give her attention. Vet stuff can come later especially if she is an indoor cat.

You are probably great for each other! I feel the same way often about my kitty--am I doing enough? He needs his teeth cleaned but I probably won't have the money till after the new year. I understand kitty guilt!