It is hard when you go through the ups and downs when you are coming closer to the point of no return. In many ways, this is the most difficult and emotionally taxing stage because of the constant uncertainty, questioning and soul-searching - and actually grieving.
That is why death itself can come as a relief you should never feel guilty about because it means that you have done a lot of your grieving homework. Having lost my dad after a drawn-out three years up and down battle with terminal cancer during which he was came close to dying three or four times has taught me that one.
You still have the pain of the loss, but much of the other emotional baggage that comes afterwards is what you are sorting through right now.
I hope that this will help to understand your current experiences and mental state better? Knowing that you have done the best you could for Enid and have gone through quite a lot of the emotional leave-taking beforehand will make things noticeably easier for you afterwards, apart from the actual pain of the loss itself. The grieving dynamics with a terminal illness are different but I can reassure you that do not have to the whole homework twice.
PS: More rescues are now willing to adopt out a companion that reverts to them upon the death of your own piggy. This means new companionship but an end of the see-saw.
From my own experiences, giving up a beloved piggy of yours means that you have to go through a grieving process tinted with a lot of guilt, even when you know that you have made your decision for the best interests of your piggy. That is an aspect most people are not aware of, and it can catch them out badly.