First of all you should examine to find out who the best spiritual father is. As Basil the Great says, people do not reveal (expose) their bodily wounds and pains to everyone but only to experienced doctors who know how to cure them. Similarly sins should be revealed only to those who can cure them... But even when you confess all your sins and hide one, out of shame, you should know that not only the sins you have confessed remain unforgiven but you also add up one more for yourself for hiding one, as is said in the confessor's book by Chrysanthos of Jerusalem. Therefore you are wisely advised by a Teacher that if you want to defeat the devil -who brings you shame for a sin - you should confess first the sin for which you feel the greatest shame. If you know how to write, brother, note down your sins on a piece of paper so as not to forget them.
You should also know that unless you properly examine your sins before confession, the sins you may have forgotten remain unforgiven through your willful forgetting. And all of this is so because you had failed to examine your sins before confession. However, if you prepare properly before confession and happen to forget a sin, being human, this sin, some say, is forgiven along with the others you have confessed because this kind of forgetfulness is considered as a involantary and not as a willful one.
If you prepare yourself and happen to forget this sin, after the confession you should return to your confessor to reveal even this sin. As we read in the patristic books a certain abba (holy ascet) could see the souls going to Hades (hell) in the same way snow comes down to earth in winter. Why was this happening? Not because Christians do not confess their sins but because they do not confess them properly without making up their minds not to repeat the same sins and also because they do not tear their hearts out a decisive pain for correction but, instead, they rend their garments, according to the prophet, falsely; "Rend your heart and not your garments" (Joel 2,13)
There is no point, my brother, in simply saying "I have sinned, I repent". This is what Saul (a ΄ Kings 15,24) and Judas said (Matthew 27,4) but it was of no avail to them. The most important thing in repentance is deciding to change your life. Do not say; "If I can, I will be better or that I would not like to sin again". Do not say that. Instead, say; "I have decided to correct myself, I do not want to sin any more in the same way I do not want to drink a cup full of poison, in the same way I do not want to fall from a precipice, in the same way I do not want to kill myself".
Confession should be made before experienced spiritual Fathers
Orthodox Kypseli Publication