disease resembling neurosis
怔 centers on a mental or emotional disturbance resembling neurosis, characterized by fear and agitation. The given meaning points to a disease-like state of nervous dread.
怔 combines the heart radical 忄 with 正, which likely contributes a phonetic element. The character is associated with a disease of the mind or emotions, but the exact historical development is uncertain.
The heart radical 忄 beside 正 suggests a heart that is not right, leading to a nervous, fearful state like a neurosis.
For セイ, imagine a person saying 'say' in a trembling voice because of their neurotic fear: 'say' -> セイ, and the heart is unsettled.