A discussion with Fuzzybear (Thanks!) prompts me to add:

This business is forcing me to refine my ideas on what causes mental illness! It can be due either to conflict with parents who try to enforce thought control, or by events such as physical/sexual abuse, either of which overwhelms a system which is required to attempt to cope with contradictory demands: the need to express and recognize one's own thoughts and feelings, and the need to gain approval of, and believe in the goodness of, caretakers. I think this could explain both "ordinary" thought censorship and more extreme cases of abuse so terrifying that the young person's system cannot bear it without splitting or having other maladaptive adjustments to the stress. It could even explain what can happen with stresses of non-psychological origin too threatening for a young person's system to adapt to -- stresses such as repeated severe physical illnesses or things like surgeries when the person can develop no explanation for why they are happening.