Euthanasia is ALWAYS a hard descison. Its tough to gauge pain/discomfert in reptiles.
Its imortant to remember that quality of life, not quantity, is important when dealing with animals.
Do you feel that she has a good quality of life?
It also comes down to how long you are able to sustain her in this state, is she losing weight, does your schedule permit the kind of nursing care that is required.
How much is she eating in regards to how much is being deficated?
Also consider the diet she is being fed, it is not meant to be long term and other complications may arise from her diet alone.
It seems her lack of deficaton is not obstructive in nature, but that she will not eat on her own.
Definately not trying to push you either way, just different factors to think about, and no one will judge you either way, it is your choice.
I wish you luck.