Allah's slave --- i.e. every one of us --- can find comfort only by turning to Allah, the Exalted, Who mentioned tranquility in many verses of the Qur'an;
(Then Allah sent down His Sakinah [calmness and tranquility] upon His Messenger and upon the believers) (Qur'an 48: 26)
(...He sent down calmness and tranquility upon them….)
(Qur'an 48: 18)
(Then Allah did send down His Sakinah [calmness, tranquility and reassurance, etc.] on the Messenger [Muhammad].....) (Qur'an 9: 26)
Tranquility means having a heart that is at peace and that has a sound trust in Allah. Tranquility is a state of calm enjoyed by those believers who have been saved from doubt. And it is according to one's closeness to Allah and steadfastness in following the Messengers way that one will achieve tranquility and peace.
(Allah will keep firm those who believe, with the word that stands firm in this world [i.e. they will keep on worshipping Allah Alone and none else], and in the Hereafter) (Qur'an 14: 27)