Change the Payment Method

I am 17 years old and I am using Family Sharing with my family member. I used to have my own payment method (WeChat Pay). For some reason, my family member removed my payment method and replaced it with his own method (Alipay). This means that whenever I make a purchase, I am actually using his money which I do not want it to happen. I want to add my payment method but it does not allow me to do so. I just stopped Family Sharing and it still does not work. How can I change my payment method?

Thank you!