Does breaking a promise I made with Allah—similar to the covenant mentioned in the verse: "And those who break the covenant of Allah after contracting it..."—make me among those included in the curse mentioned in the verse, or is only an expiation for an oath (kaffarat yamin) required of me?

A Muslim should be keen to fulfill his covenants, especially what he has covenanted with Allah regarding abandoning a sin. Scholars have differed concerning someone who says, "I covenant with Allah to do such-and-such," whether it is an oath, or a vow and an oath, or whether nothing is binding upon him. If you offer an expiation for an oath and repent to Allah the Almighty, then one who repents from sin is like one who has no sin.