Sayyid al-Istighfar is the most complete formula of seeking forgiveness taught by the Prophet (peace be upon him): 'Allahumma anta Rabbi la ilaha illa anta, khalaqtani wa ana abduka, wa ana ala ahdika wa wadika mastata't, a'udhu bika min sharri ma sana't, abu'u laka bi ni'matika alayya, wa abu'u bi dhanbi, faghfir li fa innahu la yaghfiru al-dhunuba illa anta.' The Prophet said that whoever recites it during the day with firm conviction and dies that day before evening will be among the people of Paradise, and likewise whoever recites it at night. It is best said as part of the morning and evening adhkar. It combines affirming Allah's lordship, confessing one's servitude and sin, and pleading for forgiveness.
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.