There’s no secret recipe for fluency; indeed, it’s difficult to even find consensus on what "fluency" actually means. Is it the ability to have a conversation, or language proficiency equivalent to that of a native speaker?
The distinction between tú and Ud. is one of the most confounding aspects of Spanish, and one of the most basic. The influence it has on verb conjugations and pronouns is considerable, but more than that, the choice of tú or Ud. is a matter of etiquette.