Am I a mezzo with a developed low range or a contralto?

Your voice is more of a contralto than that of a mezzo-soprano, given that your most comfortable singing range is between D3 and B4. A mezzo-soprano's comfortable singing range is usually from F4 to F5, with occasional excursions up to B5. Also, a mezzo's passagio is usually from C5 to E5, an area of your voice which you have some trouble smoothing.