Allaahumma 'innee dhalamtu nafsee dhulman katheeran, wa laa yaghfiruth-thunooba 'illaa 'Anta, faghfir lee maghfiratan min 'indika warhamnee 'innaka 'Antal-Ghafoorur-Raheem.
O Allah, I have greatly wronged myself and no one forgives sins but You. So, grant me forgiveness and have mercy on me. Surely, you are Forgiving, Merciful.