Formal:
* Ma Princesse: This is the most direct and formal translation.
* Votre Altesse: This is a more respectful and formal way to address a princess, especially if you are speaking to her directly.
Informal:
* Mon Amour: This translates to "my love" and can be used affectionately for a woman you love, even if she is not a real princess.
* Ma Chérie: This means "my darling" and is a very sweet and endearing term.
* Mon Ange: This translates to "my angel" and is a very romantic way to refer to someone you adore.
The best way to say "my Princess" in French will depend on your relationship with the person and the context of the situation.