Unable to add payment method having Family Group / Purchase sharing

A friend of mine added me to his Family Group at Apple; he is the organizer of this family group. The problem is that I now cannot use my own payment methods due to "purchase sharing" being enabled and being set at default with an old payment method of my friend's. I need to be able to use my payment method while still maintaining the shared subscriptions in the Family Group. Is there a way to use my payment method without losing the shared subscriptions? It should be clear that I wish for my payment method to be only mine: as much as we are in a Family Group, I am an adult and I should be able to make purchases with a method that only I can use; it is my card after all.