Allah Says in Quran: There is no fear on the humble servants of Allah, nor sadness.(Sura Yunus, verse 63).
It is good to think about the great Mercy of Allah and repeat Ya Arhama Arrahimeen and Ya Rahman Ya Raheem.
One of the best ways to find peace in our heart is by repeating Salawaat (ALLAHUMMA SALLI ALA MUHAMMAD WA AALI MUHAMMAD) as well as Esighfaar (ASTAGHFIRULLAH RABBI WA ATOUBU ELAYH).
Wassalam.