Here’s a lovely resource I enjoy with a short and sweet description of co-dependence: Emotional Sobriety by Tian Dayton
She describes how some children learn from a young age to take care of parents. Sometimes the children become “small”, in the sense of not attracting attention, or trying to be less visible or needy. They feel that behavior creates less burden to their parents, thinking that keeps parents from going over the cliff. If parents aren’t able to pay attention to their child, because of the parents’ own issues, the child can develop a hyper vigilance. Often children influenced this way tend to take care of others and lose themselves in the process.