I am so sorry. It does not matter when you reach this point, how old your mother is.
What does matter is the fact that she has always been there and now she soon will not be there.
This hurts.
You ask is it time to let her go? Honestly, no one can answer that question for you.
My mother was in a coma for a month before she died. One day when I visited I held her hand and said, "If you want to go, Mummy, do so - M (my sister) and I will look after Daddy." I knew she had always worried about how he would cope if she died first (He coped well, as I had known he would). I could sense no reaction to my remark, but the following day she had opened her eyes and recognised my sister, then died three days later.
So perhaps your mother needs the reassurance that you are a big girl now, and can manage alone.
If she is suffering, speak to the doctors or the nursing staff and say clearly that you feel the important thing right now is to make sure your mother is in no pain, or as little pain as possible.
This statement will be understood to mean that you will not complain if an increased dosis of morphia shortens your mother's last days or hours.
Your feeling of numbness will pass, but right now it is probably preferable to feeling the need to weep constantly.
Try to think that your mother has lived a long life, and reached the point where life is becoming a burden to her.
Whatever your and her relationship has been, your post shows me clearly that you love her, and I have no doubt she loves you too. Try to remember the good times and don't blame yourself for the differences you and your mother have had.