I think that what you have described about coming to terms with your mom dying is a rare and special gift and should not be looked upon as being forbidden. Not all people are spiritual and so cannot understand the link that you have with your mom that goes beyond life and death.
The thought of my mom dying (and other family members) was one of my worst fears even from when I was a child. This fear held so much control over me for such a long time and probably contributed to my illnesses, but such feelings of fear or sadness cannot help you in preparing for the death of a loved one. When she died suddenly I was destraught and it took me a long time to come to terms with her death.
I hope that your mom's remaining time here is peaceful and pain free.