People like to say death is uncertain but the only part of life we can be truly certain of is death and most of us are not ready for it. That certainty becomes more real when you are presented with facts that prevent you from ignoring reality, but knowing the inevitable will happen does not make it hurt any less. Losing a loved one is nothing but pain, but grieving a loved one that is still living and accepting they are slowly dying is, in my opinion, worse.
There is no right way to prepare for it. Fear becomes a part of your life, and hyper-vigilance takes over, thinking the next call you get will be the news you never wanted to hear. It becomes more difficult when that loved one lives miles away and you cannot see them in person ever again. You will experience sadness, anger, denial, guilt, frustration, you name it. But you just have to own it and cope with it the best way that you find because if anticipatory grief goes on for a long time, it will wear you down emotionally and physically.
I found the way for me to own it and accept it was through writing, because is the best way I know how. Consider a method to express your pain whether that is asking for help or telling someone how you feel. Spend time with that person if you can. If you don’t live near and can’t visit, talking on the phone with that person also helps. Find closure and help your loved one find theirs.