yes.  as someone who has been on that journey, and who
fortunately had a mother with the good sense to sign a living
will saying that she didn't want any life prolonging measures,
i'll be extremely blunt.  her death was a mercy for her and for
her family.

extending the life of a person with a severe dementia is not kind
or helpful.  why would anyone want to extend the life of someone
who is confused, frightened, and unable to cope?  it's simply
cruel to extend the life of someone who has no hope of recovery.

were you keeping her alive for you, or for her?  sometimes you
simply have to recognize that all things end and let your loved
one go.  

sometimes the grief of dealing with a dying person takes over
one's life, and you can't let go.  if so, you need to call your
local hospice and ask about grief counseling.  most hospices
offer it, and it's usually available for or at a nominal charge
based on your income.