((((Mary))))
I do think most of us expect our parents to die some time or other, and being deceived about reality probably is taking its toll on you. I was advised on another thread to just try to enjoy the time left with the family that remains, and this is also applicable to your situation. Try to appreciate the fact that they're still living and be grateful that they have lived so long. Then think of ways you can add to their comfort and well being while they're still around.
It's hard to know what to do with those feelings you're having. Have you talked to a counselor/therapist? Dr. Elizabeth Kubler-Ross wrote a book called "On Death and Dying" some years ago which has been helpful to many. But don't read her more recent works, as she has gotten weird, talking to beings from other dimensions and such.
Do you have siblings you can share your feelings with? If not, do you have close friends who will understand? Don't suffer alone. Ask for help if you need it.
Take care,
Sandra