I know exactly what you mean. I used to feel the same way about my sister. I'd see her.....but it really wasn't "her".....just the body that she used to live in. She was in pain, having zero quality of life and just existing.
I used to feel so guilty for praying and asking the Lord to just please take her. I only wanted her suffering to be over. But I still felt horribly guilty for it.
It's so hard to watch a loved one suffer and know there is nothing you can do to help. One day we will know "why" but for now all we can do is have faith and trust that He is in ultimate control.
You and your Mom are in my prayers.