Is what I pledged to myself considered a vow (nadhr) or an oath (yameen)? And what are the consequences in case of inability to fulfill it, or if the action is repeated?

What you uttered is considered an oath to abandon sin, and it must be fulfilled. If you break your oath by committing the sin, you incur a sin and repentance becomes obligatory upon you. You are then obliged either to fulfill what you swore upon (giving fifty pounds in the first instance, and fasting seven consecutive days in the second), or to offer expiation for an oath (feeding ten needy persons, or clothing them, or freeing a slave; if unable, then fasting three days). If you expiate for your oath, it is dissolved, and no further expiation is required unless you swear another oath, or if the wording or intention necessitates repetition.