There are plenty of synonyms for the common phrase "that was silly of me". Some options include "that was foolish", "that was ridiculous", "that was senseless", "that was imprudent", and "that was ridiculous". You could also use phrases like "what was I thinking?", "I must have been out of my mind", or "I should have known better". These alternatives can convey a similar sentiment of acknowledging a mistake or error in judgement, but with slightly different connotations or tones. Whether you choose to use a synonym or the original phrase, it's always important to take responsibility for our actions and learn from our mistakes.