If you are in doubt whether to write a letter or not, don't. And the advice applies to many doubts in life besides that of letter writing.

Bulwer-Lytton’s counsel treats hesitation as a warning signal: if you feel uncertain about sending a letter, the safer course is restraint. Letters (in his era, the main durable medium for private communication) could be preserved, forwarded, or quoted, turning a momentary impulse into a lasting liability. The second sentence generalizes the principle into a broader ethic of prudence—when motives are mixed, emotions are hot, or consequences are unclear, inaction may be wiser than action. The remark also implies a moral psychology: doubt often indicates that one’s judgment has already detected a risk (to reputation, relationships, or integrity) that one’s will is trying to override.