Am I obligated to offer expiation (kaffarat yamin) for oaths I made with Allah to refrain from specific sins, then I reverted and committed them, knowing that I was unaware that phrases like "I covenanted with Allah" carry the ruling of a vow (nadhr), and I do not remember the number of times these covenants were made by me?

Whoever makes a covenant with Allah is obliged to fulfill it. If he breaks the covenant, he must repent, and a number of scholars hold that an expiation for breaking an oath (kaffarat yameen) is also required if he vocalized the covenant, such as saying: "Upon me is the covenant and promise of Allah that I shall do such and such." However, if the covenant was merely a thought not vocalized, or not attributed to Allah, then nothing is incumbent upon him.