Here is one I like that may help you…I assure you the prayer works:
Receive my confession, O most loving and gracious Lord Jesus Christ, only hope for the salvation of my soul. Grant to me true contrition of soul, so that day and night I may by penance make satisfaction for my many sins. Savior of the world, O good Jesus, Who gave Yourself to the death of the Cross to save sinners, look upon me, most wretched of all sinners; have pity on me, and give me the light to know my sins, true sorrow for them, and a firm purpose of never committing them again.
O gracious Virgin Mary, Immaculate Mother of Jesus, I implore you to obtain for me by your powerful intercession these graces from your Divine Son.
St. Joseph, pray for me.
I have been guilty of hiding sins in my past, or not properly meditating on my soul in discernment to search them out, I pray that you will not do as I had done. Hold nothing back, for then your penance will not be valid.(Remember, you are talking directly to Jesus, the Priest is only his vessel)
The actual formula can go something like this.(not necessary, but may help you)
Enter the confessional, make the sign of the cross and say "Bless me Father for I have sinned, it has been (state the length, say 20 years) since my last confession. "
Confess your sins, being very careful to tell him the exact(or as close as you know) number of times of each occurrence.
At the end, you can add, "for these sins, and those I have forgotten, I am truly sorry"
The priest will then take over, giving you advice, stating your penance and then will ask you to say an Act of Contrition. (If you dont know the Act of Contrition, tell the priest and he will guide you through it.
The Priest will absolve you of your sins, then you can say, "Thank you, Father."
Then go and perform your penance.