What is the ruling on one who doubts whether they committed a sin or doubts their sincerity and intention, and is it obligatory upon them to repent from something they doubt having done?

A servant is commanded to repent to Allah in all circumstances, even after performing acts of obedience, so as to compensate for any deficiency, just as we are commanded to seek forgiveness after prayer, Hajj, ablution, and night vigil. If one doubts whether they committed a sin, the default assumption is that they did not commit it, and one should not open the door to intrusive thoughts in this regard. A servant is commanded to attain sincerity (ikhlas), and having a negative self-perception is good if it leads to perfecting sincerity and monitoring the heart. However, if it reaches the point of debilitating fear that hinders one from performing good deeds, then it is blameworthy fear. The ultimate knowledge concerning the reality of one's state and whether an action is sincere belongs to Allah the Almighty, and the servant lives between hope and fear.