As a parent, I scoff at the airplane mask and filling your pitcher metaphors, because my heart says it's ludicrous to focus on me and my needs when my child has needs. However, as a psychologist working with young people and their families, the number one piece of advice I find myself giving to parents of youth who are facing incredibly challenging situations is to manage their own well-being first. In short, the advice I give, is to go against your heart in this moment. Take the time you need so that you can be what your child needs. A child who is scared is not going to benefit from a parent who is so stressed that they can't hold their child's fear for them. Your child needs you to be their strength. They need to know that you see them as more than their illness, that you are strong enough to hear them say they are scared or in pain. If you are stuck in your own pain and distress and stretched thin because you haven't taken care of yourself physically or emotionally, there is no way you can be the support system your child needs. Go against that immediate pull at your heart, put on your mask, and fill your pitcher so you can be the parent your child needs.
Your child is not her illness, and the illness is not your child. Children with chronic illness are, first, children. They need to play and grow and do child things, maybe even more than health-typical kids. Some health problems might make some activities problematic, so managing this takes creativity and a willingness to take chances. It's not always easy.