Nancy, first my I say I am so sorry you are going through this sad and painful time in your life. Losing a mother is so hard and of course, can be quite painful.
Mom's deterioration was caused by her declining illness. People who go on Hospice usually are on the program because medicine can no longer help or cure a person.
Hospice if a life giving organization in that it helps to enhance quality of life, offer support to patient and loved ones and of course make sure a patient is comfortable. It sounds as though your mother is suffering and of course anxious probably due to her inability to breath well. This is normal. So is her disease process. It may seem odd, but her decline indicates that she is very sick and in need of hospice care. Hospice provides spiritual as well as emotional support, which you might want to take advantage of as soon as possible. Your feeling of helplessness is also caused by fear of losing you mother and death.
All normal...as is your feelings. You are a wonderful daughter whose love and deep concern comes through in your plea for support. What you can do for your mother is spend time with her, care for her and let her know that you love her. Also let her know you will be ok.
Ask questions of Hospice and of course if you have a clergy person you can go to...friend...do so. You really need support now as well. My best to you and my prayers to you and of course, your mother. May she find peace, no pain and continued love.