I see your point; we usually say "true to yourself", though. "Unfaithful to yourself" is not idiomatic. I can possibly imagine saying "faithful to myself, I did ABC", but "unfaithful to myself" is simply too stunted.
Language, unlike some body parts, benefits from smaller size. That might be even one of the reasons English, with its short words, is the modern lingua franca.

"true to yourself" wins over "faithful to yourself" on brevity, and wins big time - the ratio of the number of characters is 1 to 2