Please don't blame yourself, as you're not the vet and can't be expected to know about all of these drugs out there. You're right about the kidney thing just coming on once it's too late. With Minnie she just got really sick one day, we gave her IVs for 2.5 days, kept her extremely hydrated with 24 hour care, had her into the vet or the ER almost every day, and she was still gone within the week.
And yeah, I doubt you'll ever get over losing Promise; I know I never will with Minnie. But it's not your fault --- you were being the responsible owner in making sure your pup was inoculated against heartworms, fleas, and the like. It's not your fault; it sounds like it may be Lilly's fault.