Does the statement: "I prepare myself by resolving that I will not do such and such" fall under the rulings of a vow (nadhr) or an oath (yameen), even if the intention is to make a firm decision and not an explicit promise to Allah?

Oaths and vows represent a commitment to the occurrence of a specific action linked to God. A vow is considered a type of oath, so the statement "It is a duty upon me for the sake of Allah to do [such and such]" is similar to "I swear by Allah that I will do [such and such]" in its commitment to an action linked to God. The phrase "I pledge to Allah" is also a form of vow and oath; it is a vow, a covenant, and an oath. As for "pledging to oneself," it is not a form of vow or oath, but rather an expression of a person's determination and resolve to act.