All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
There are two probabilities to this promise (vow) which you mentioned.
1- You made this promise with an expression of undertaking a vow, like if you said for example: “I have to complete the recitation of the Quran for the sake of Allaah if He cures me from the whispers.” This is a vow of recompense, i.e., doing something for Allaah The Almighty in return for something that He has fulfilled for you. Thus, you are not obliged to fulfill this vow unless you are cured of the whispers. If you are cured, you are obliged to fulfill it within the time that you specified. If you fail to finish reciting the whole Quran within this specified time, then you are obliged to make expiation for breaking your vow.
2- If it was just a promise and not a vow: The ruling of promising to Allaah The Almighty is like the ruling of an oath, if one does not fulfill it, he must expiate for it. For more benefit on expiation for breaking a vow, please refer to Fatwa 102811 and 82101.
Finally, it should be noted that the vow that is undertaken as a due recompense when Allaah The Almighty fulfills something for you is disliked, as the Prophet disliked it and said: “Making a vow does not bring any good but it only extracts something from the stingy people (causes them to spend from their money).” [Al-Bukhari and Muslim]
For more benefit, please refer to Fataawa 90796, 90339 and 89891.
Allaah Know best.