There are a number of authentic duas/prayers for astaghfar (repentance), both from Quran and hadith. Doing astaghfar is your regular prayer and one should never put off turning to Allah for their mistakes.
Dua # 1
Dua # 1
استغفراللہ ربی من کل ذنب واتوب الیہ
"I seek forgiveness for my sins from Allah who is my Lord and I turn towards Him (for repentance)"
Transliteration: Astaghfirullaha rabbi min kulli zambin wa-atoobu ilaih
Dua # 2
رب اغفر وارحم وأنت خير الراحمين
"My Lord, forgive and have mercy, and You are the best of the Merciful"
Transliteration: Rabbighfir warham wa-anta khairur rahimeen
Dua # 3
استغفراللہ الذی لاالٰہ الا ھوالحی القیوم واتوب الیہ
"I seek forgiveness from Allah, there is no God but He, The Ever-Living, and the One Who sustains, and I repent to Him"
Transliteration: Astaghfirullahal lazi laa ilaaha illa hual hayyul qayyumu wa-atubu ilaih
Dua # 4
ربنا ظلمنا أنفسنا وإن لم تغفر لنا وترحمنا لنكونن من الخاسرين
"Our Lord, we have wronged ourselves, and if You do not forgive us and have mercy upon us, we will surely be among the losers"
Transliteration: Rabbana zalamna anfusana wa-in-lam taghfirlana watar hamna lanakunanna minal khasireen
Dua # 5
لااِلہ اِلا اَنتَ سُبحانک اِنی کُنت مِنَ الظلمِین
"There is no God but You. Glory be to You ! Truly I have been of the wrongdoers"
Transliteration: La ilaha illa anta subhanaka inni kunntu minaz-zalimeen
These are some of the most popular duas (prayers) for astaghfar (repentance) mentioned in Quran and hadith. You must make these duas part of your daily recitation as we need to be on guard all the time, making sure Allah is not angry with us due to our intentional or unintentional mistakes/sins.
I have added English transliteration for those of you whose first language is not Arabic. Although arabic text is important as we have to memorise these prayers in the original language, but understanding the real meaning of these duas are all about is even more important as Allah wants us to repent with all our heart - not just some empty words with no real feelings of guilt and shame.
Stay blessed and thanks for dropping by. Please share these duas with as many people as you can.