Your predicament brings The Serenity Prayer to mind. Many people in powerless situations, whatever their view of religion or whether they embrace any religion at all, report that simply repeating
this prayer to themselves offers some solace:
(God) Grant me the serenity to accept that which I cannot change
Courage to change the things I can
And the wisdom to know the difference.
There is so little in your mother's situation that you have the power to change, and this may be the source of your discomfort. It is no fun to be face-to-face with powerlessness. It takes most people to a position of wanting to do something, to take some sort of action, to help stop feeling so out-of-control.
But it sounds as if you have done all you can do to assuage your mother's misery. She is in hospice and adequately medicated, which may realistically be all that can be done.
If you can't reason with your mother, then it would probably be futile to try to talk her into viewing her demise any differently, or processing her feelings about her impending death.
You might be well-served, however, by talking with someone about these feelings of powerlessness. Hospice volunteers are very often a terrific resource. Try to talk with one of them about your feelings. If that conversation doesn't relieve your feelings, he or she may know of someone in your community who will be helpful.
And if The Serenity Prayer doesn't "speak" to you, devise your own prayer or phrase to provide comfort to you as you witness the end of your mother's life.