Will a person be held accountable for an action if it was based on a fatwa that later proved to be incorrect?

What you are experiencing are mere whisperings [waswas], which you must repel and disregard, lest they corrupt your life. Ibn Hajar al-Haytami said about the ailment of whisperings: "It has an effective cure, which is to completely ignore them, no matter how much doubt there is in one's mind. For if one does not pay attention to them, they will not endure, but will disappear after a short time. As for him who listens to them and acts upon their dictates, they will continue to increase until they drive him to the realm of the insane."

Even if the fatwa (definition) is incorrect, you are not sinful, nor is the mufti (definition) sinful for his ijtihad (definition), for he is excused if he did not fall short in his ijtihad. If he did fall short, the sin is upon him, not upon the questioner. Al-Qari said in "Mirqat al-Mafatih": "Meaning: If every ignorant person asks a scholar about an issue, and the scholar gives him an invalid answer, and the questioner acts upon it without knowing its invalidity, then the sin is upon the mufti if he fell short in his ijtihad." So, abandon these whisperings and do not give in to them.