أَسْتَغْفِرُﷲَ الَّذِى لآإِلٰهَ اِلَّا هُوَ الْحَىُّ الْقَيُّومُ وَ أَتُوبُ اِلَيْهِ
AstaghfiruLlaah-alladhee laa ilaaha illaa huwa-al-ḥayyul-qayyoomu wa atoobu ilayhi.
I ask for forgiveness from Allah, there is no deity-god, only the One who is Hayy and Qayyum. My repentance is to Him!
The Rasul of Allah (sa) says: “He who says, ‘I ask for forgiveness from Allah, there is no deity-god, only the One who is Hayy and Qayyum. My repentance is to Him!’ will be forgiven even if he has fled from battle.”
Two important points to consider are, using the Ismi Azam while repenting and that even major mistakes are forgiven with such repentance.
I will cover the wisdom behind using the Ismi Azam when praying in the section titled Ismi Azam. As for forgiving even those who may have fled from battle... Fleeing from battle is one of the seven major sins according to the Rasul of Allah (saw).
He says, “Stay away from the seven things that cause destruction:
Assigning partners to Allah (duality)
Killing someone who Allah has forbidden to kill
Engaging in black magic and usury
Consuming the property of an orphan
Fleeing from battle
Slandering a chaste woman with adultery.”
Therefore, forgiveness encompasses the major sins also, and yet to be forgiven one needs not confess to someone as the Christians do; one can directly turn to the Sublimity and Greatness of Allah and confess his mistakes to Him and ask for forgiveness from Him alone.
This being the case, no matter how great our mistakes and sins may be, let us never lose hope, let us turn to Allah and not postpone repenting.
أَللّهُمَّ اغْفِرْلِى ذَنْبِى كُلَّهُ وَ دِقَّهُ وَجِلَّهُ وَ أَوَّلَهُ وَ اٰخِرَهُ وَﻋَﻼَنِيَتَهُ وَسِرَّهُ
Allaahumma-ghfirleedhanbee kullahu wa diqqahu wa jillahu wa awwalahu wa aakhirahu wa `alaa niyatahu wa sirrahu
O Allah, forgive all of my mistakes, the old ones and the new ones the small ones and the big ones, ones that I have done openly and ones that I thought of.
This is one that the Rasul of Allah (saw) most frequently recited...
Note the scope of his repentance. Like I said before, it is imperative that we not repeat these as a parrot would, but approach things the way the Rasul of Allah (saw) did...
There is also great benefit in reading this istighar before giving salam in salat.