Salam ilker
Allah has said that He has created humans as weak. And also to toil and struggle. I do not know what the nature of your sin is but Allah is the most merciful, have faith in Him that He will judge you based on ur intentions and actions. We are weak and we sometimes succumb to temptations even when we vow that we would not repeat that particular sin again. But that happens. Pray for strength and forgiveness.
Depending on the nature of your promise as in promise to not do what! There are things that can be done as expiation. Options are given
5:89 yusuf ali
God will not call you to account for what is futile in your oaths, but He will call you to account for your deliberate oaths: for expiation, feed ten indigent persons, on a scale of the average for the food of your families; or clothe them; or give a slave his freedom. If that is beyond your means, fast for three days. That is the expiation for the oaths ye have sworn. But keep to your oaths. Thus doth God make clear to you His signs, that ye may be grateful.
5:89 muhammad Asad
GOD will not take you to task for oaths which you may have uttered without thought, but He will take you to task for oaths which you have sworn in earnest. Thus, the breaking of an oath must be atoned for by feeding ten needy persons with more or less the same food as you are wont to give to your own families, or by clothing them, or by freeing a human being from bondage; and he who has not the wherewithal shall fast for three days [instead]. This shall be the atonement for your oaths whenever you have sworn [and broken them]. But be mindful of your oaths!' Thus God makes clear unto you His messages, so that you might have cause to be grateful.
5:89 sahih international
Allah will not impose blame upon you for what is meaningless in your oaths, but He will impose blame upon you for [breaking] what you intended of oaths. So its expiation is the feeding of ten needy people from the average of that which you feed your [own] families or clothing them or the freeing of a slave. But whoever cannot find [or afford it] - then a fast of three days [is required]. That is the expiation for oaths when you have sworn. But guard your oaths. Thus does Allah make clear to you His verses that you may be grateful.
So whatever among these options is feasible for you. But the point is was it something you knew that was pronounced without much thought? The thing is that it is only you who can decide after contemplation that whether you need to make expiation or just praying for it will suffice.
Good luck