Heidiangel to reiterate what William said: You chose the best vet, Heidi had come out fine before, you had no reason to second-guess this one, or the vet's abilities. It is the most confusing and difficult thing to accept, but sometimes...things just happen. I do not by any means intend to sound flippant and if it comes off that way I apologize profusely.
My Dante had begun to respond positively to his meds. His last day with us before his little heart failed, he'd been the most energetic and himself he'd been in weeks. The next morning was such a shock, it just all came crashing down on us. I had done everything I was supposed to, the vet did everything to help, he seemed to be picking back up but...it just happened, anyway. Its nobody's fault, its certainly not yours.
Please do not beat yourself up. You were a very diligent and loving mama.