According to rahyafte (the missionaries and converts website):It comes when remembrance of God prevails in one’s thoughts, emotions and behaviors.
The Holy Quran says in Verse 28 of Surah Ar-Ra’ad: “And the faithful ones whose hearts are comforted by the remembrance of God. Remembrance of God certainly brings comfort to all hearts.”
From Islam’s viewpoint, the highest and loftiest human status is achieved when one is in the state of remembering God and divine light shines in his heart. It is in such a state that one’s mind reaches peace and tranquility.
God says in the Quran: “One whose chest (heart and mind) is left open for Islam (submission to His will) shall receive light from God. Woe to those whose hearts have become like stone against the remembrance of God. They are clearly in error.” (Verse 22 of Surah Az-Zumar)
That is why when there is a tragedy or adversity, people are advised to say, “Surely We belong to Allah and to Him we shall return.” (Verse 156 of Surah Al-Baqarah)
So one important step toward achieving mental health, healing mental issues and problems, and reaching mental peace and certainty is remembrance of God.
Also, Dhikr (reminding oneself and praying) is a way to heal negligence and move on the path of growth and burgeoning of human aptitudes. Hence, in order to reach peace in life, we should pray to God and take recourse in Him. This prayer itself creates peace in mind because praying is among the examples of remembrance of God.
In Verses 83-84 of Surah Al-Anbiya, God refers to the story of Prophet Job (AS) and says, “When Job prayed, ‘Lord, I have been afflicted with hardships. Have mercy on me; You are the Most Merciful of those who have mercy.’ We answered his prayer, relieved him from his hardships, brought his family (back to him) and gave him twice as much property as that (which was destroyed). It was a mercy from Us and a reminder for the worshippers.”
