Sounds like your Mom is having the experience that is appropriate for her, as are you. However, you can help with the fear and depression by being present and bearing witness to it. Often this is done by acknowledgeing the fear, breathing in time with her, holding her hand, massageng feet, etc.
Use binary communication if you need a response. 'Squeeze my hand if you are afraid' or thirsty or in pain, are examples of this. Conversation may require more energy than your Mom has right now and as she engages her dying there is less and less interest in the outside world - normal in my experience.
Assuming she is not in pain, there is nothing to do except to be a compassionate presence.
Dale
http://companioningeol.blogspot.ca/