Boundaries of parental care
Yes, that's the hard part. We, the parents, want more than anything in the world to ensure that our children are doing well. If they get hurt, we kiss them for comfort. If they have troubles, we help them to sort them out. If they are sad, we embrace them. If someone offends them, we interfere.
But sometimes children come across really serious things that we can not solve for them. And the consciousness that we can not help anything is really terrible. Few things in the world can be worse than watching how your child is suffering, and being unable to do anything. But it happens. If someone dies, we can not bring him back, no matter how much the child does not miss him. Sometimes children fall ill so that you can not alleviate his suffering. Or one of the parents simply leaves and can not be with the child when he wants it.
This is an important lesson that children should learn: sometimes things happen that no one can do anything. This is a hard lesson, which is not easy to pass at such a young age. And when you watch it, your heart can really break. And yet it's inevitable, they have to go through this sooner or later, and you can not affect when that happens. You can only be close and somehow support the child, but you can not ease his pain.
So this rule is just that you have to learn to accept what you can not change. It's not your fault, and no one else in your place could do better. So you do not need to tear your hair, you do not deserve it. And so everything is not too easy for you, right? It is possible that you are experiencing the same pain as your child, and you absolutely do not need to pile on yourself an extra burden of guilt. It's better just hug yourself and sympathize.
And remember that children in general do not also expect miracles from you. They are not so stupid as not to understand where you are powerless. All that you can and should do for them at such moments is to give them love and sympathy, so that's what you limit. And most likely, both of you will become a bit easier.