Dua for Forgiveness of Past sins, Future sins, Sins committed in secret (private) and in public, Sins committed in Seriousness and in Fun, errors, ignorance, immoderation,

Abu Musa (May Allah be pleased with him) reported: The Prophet (sallallaahu ’alayhi wa sallam) used to supplicate: "Allahumm-aghfir li khati'ati, wajahli, wa israfi fi amri, wa ma Anta a`lamu bihi minni. Allahumm-aghfir li jiddi wa hazli, wa khata'i wa `amdi, wa kullu dhalika `indi. Allahumm-aghfir li ma qaddamtu wa ma akhkhartu, wa ma asrartu, wa ma a`lantu, wa ma Anta a`lamu bihi minni. Antal-Muqaddimu, wa Antal-Mu'akhkhiru; wa Anta `ala kulli shai'in Qadir (O Allah! Forgive my errors, ignorance and immoderation in my affairs. You are better aware of my faults than myself. O Allah! Forgive my faults which I committed in seriousness or in fun deliberately or inadvertently. O Allah! Grant me pardon for those sins which I committed in the past and I may commit in future, which I committed in privacy or in public and all those sins of which You are better aware than me. You Alone can send whomever You will to Jannah, and You Alone can send whomever You will to Hell-fire and You are Omnipotent).''
[Al-Bukhari and Muslim].
Commentary:  The Prophet (sallallaahu ’alayhi wa sallam) was sinless but still he prayed earnestly to Allah for the forgiveness of his shortcomings. In fact, he did so to educate his followers. This prayer is an expression of one's utter humbleness and sincere repentance of sins.