I once swore that if I committed a certain sin, I would fast.

Abandoning sin is obligatory, and swearing an oath to abandon it further emphasizes its abandonment. This oath must be fulfilled, and one must not break it. If the one who swore the oath breaks it by committing the sin, he has sinned and must repent. He then has the choice between fasting, as mentioned in his oath, and performing the expiation for breaking an oath, which is outlined in the words of Allah Almighty: "Its expiation is the feeding of ten poor people from the average of that which you feed your [own] families or clothing them or the freeing of a slave. But whoever finds not [the means to do so] - then a fast of three days. That is the expiation for oaths when you have sworn. And guard your oaths." (Al-Ma'idah: 89).