How can a Muslim be sure that they are upon the correct creed, especially since followers of other religions and deviant sects within Islam display a steadfastness and belief that brings them psychological comfort? And is psychological comfort an indicator of the correctness of a belief?

Wasa'wis (whispers/obsessive thoughts) are from Satan and occur when Satan is unable to deter a servant from their religion. The Prophet praised the Companions for rejecting waswasah.

Sheikh al-Islam Ibn Taymiyyah said: Waswas afflicts everyone who turns to Allah. The servant must be firm, patient, and consistent in dhikr (remembrance of Allah) and prayer, for the stratagem of Satan is weak.

As for the psychological comfort of disbelievers, it is false. The measure of sound creed is not known by psychological comfort, but rather by what is stated in the Book of Allah, the Sunnah of His Messenger, and the methodology of the Righteous Predecessors.

The feeling of distress is relative. And whoever turns away from the remembrance of Allah will have a ضنك (ḍank) livelihood in this world and the Hereafter.

Ḍank is tightness in the chest, anxiety, confusion, and doubt, and it may also be the torment of the grave.

Strive in acts of obedience, distance yourself from waswas, and acquire beneficial knowledge.