A person who is constitutionally incapable of being honest with themselves typically struggles with deeply ingrained psychological or emotional barriers that prevent them from confronting difficult truths. This could be due to:

1. Denial: They might avoid acknowledging painful realities or uncomfortable aspects of themselves as a defense mechanism to avoid emotional distress.
2. Fear of Vulnerability: Honesty with oneself requires facing fears, mistakes, and imperfections, which can feel threatening to those who fear being vulnerable.
3. Low Self-Awareness: Some people lack the ability or willingness to engage in self-reflection, which makes it difficult for them to see their own flaws, biases, or emotions clearly.
4. Cognitive Dissonance: When their actions and beliefs are in conflict, they may unconsciously distort or rationalize their behavior to maintain a sense of internal consistency, avoiding uncomfortable truths.
5. Narcissism or Ego Defense: Those with inflated egos or narcissistic tendencies may reject any self-criticism to protect their self-image.
6. Trauma or Psychological Wounds: Deep emotional wounds from past experiences might make it too painful for someone to confront certain truths, leading them to develop mechanisms that prevent self-honesty.

In essence, such individuals have developed coping mechanisms that protect them from emotional discomfort but hinder their ability to be honest with themselves.