What title should I use for myself if I am unmarried or divorced?
In English I use "Ms.", to distinguish that I am not Mrs. (married) nor am I young and never married (Miss).
«Señora» (Mrs.) is nowadays used quite freely when respectfully addressing someone, regardless of the marital status.
The Fundéu states that, in commercial and profesional correspondence, the title «señorita» (Miss) is avoided, but it is still quite rooted in retail and education (even for married women).
Actually there are still people who will correct you to «señorita» if they are unmarried and you call them «señora», but my perception is that the distinction is falling into disuse, at least in Spain.