I completely understand what you are going through... I lost my childhood dog last year and I still cannot get those last few minutes out of my mind. We took her to the vet and spent 30 minutes cuddling her, but the shot made her shriek and I can't help but feel guilty for the pain she was in... seconds before she passed
I don't know if I could do it again after going through that.
I am so very sorry that you are going through this... We are all here for you.