In my opinion, your loss and grief is mutual exclusive to what was best for your person. The way I see it, your grief is about how her death (and her absence) impacts you directly and the "better ending" (or the awareness that she did not suffer more) is how it impacted her (empathy/sympathy). You not wanting her to have her live through what could have been a worse scenario does not mean that you don't miss her any less or that a hole was not created in your life.