Guilt is the fruit of a gift from the Holy Spirit called auxiliating grace, a temporary grace that casts light on your soul so that you may be aware of the committed sin and repent.
However, many holy authors of books on the devout life teach us that we ought not to be deeply disturbed. What is so surprising about your sins? Do you not know that without the help of the Holy Spirit we cannot even call God “our Father” (Rm 8:15)?
As you go through the proper sections of the Catechism - I exhort you to do so - in order to learn not so much about this sin but about the
virtue of chastity (which is not to repress the natural and good desires and attractions that we all have, but to find a balance and an equilibrium that is healthy for us and pleasing to God), you will learn that
chastity demands an apprenticeship in self-mastery, much like an athlete needs to train and keep a discipline if he wants to achieve something.
There is a very important thing that as a young man you should always keep in mind: when you confess, the priest is specifically acting
in persona Christi, which means that during the Sacrament of Reconciliation
you are confessing directly to Jesus Christ. Now Christ already knows all your sins - that is obvious, and in fact it will greatly aid you if you keep in mind that God is always present, though we may not see Him except through the veil of faith. When you are tempted, run away from the source of the temptation, kneel, and place yourself at the presence of God by meditating on the absolute truth of God’s omnipresence: He is by your side, in fact, in your very heart. Even after you sin mortally, God is still by your side, in fact even more actively seeking your repentance.
So when you confess, do use the veiled window or any other means that make you feel more comfortable, but forget about the fact that you know the priest: at that very moment, it is to Christ that you speak. It is not a man he who tells you: “I absolve you”, but the Divine Judge Himself.
At this age is the time in which the ancient adversary will be particularly active, and the Lord looks forward to sanctify you and make you holy. One thing the adversary does is to keep people away from confession - he hates confession! - and above all to keep them from receiving Christ in the Holy Eucharist.
You should confess very often. Possibly weekly. Confession is not only for mortal sins. We are open and actually encouraged to confess all sins, even venial ones, and even imperfections in our self and temptations that we perceive. Confess often, so that Christ may fill you every time with greater sanctifying grace. Remember: we cannot do anything on our own, but Christ has the power to make us holy, and to do so in an instant.
If you want to be a brave spiritual warrior, then you must become humble and imitate our Lord who washed with his hands the feet of his disciples to purify them. Your embarassment is a profound sign of pride, which is not just a sin, but a root sin, origin of many other sins - among which, possibly, even the very sin against chastity that you are struggling with…eradicate pride from your heart by becoming humble, like Christ, who being the Creator of all things and King of the Universe did not hesitate to say to the Father: “
I am your servant, o Lord…your servant and the son of your handmaid”.
Go and do the same.
Before confession, possibly the night before, do an examination of conscience.
This may help you.
When you go to confess, you may simply close your eyes and talk to Jesus openly, with simplicity. You are His son, His brother, His friend. Tell him that you feel guilty and that you are sorry, partially because your pride is hurt, partially because you have offended Him. Tell him the mortal sins you are aware of. Tell him about any other sin that you may be aware of, regardless of whether you consider it venial. Tell him about the temptations, especially the worst ones, which are the ones you mention here. Temptations of lust are quite common and normal, and may come from our mind, but temptations against confession come straight from the adversary. Tell him anything you need to tell Christ. Then be still and listen to the response and the advice that Christ will give you through his servant, pay careful attention to the penance that the Divine Judge will impose, and receive the Holy Spirit and sanctifying grace, remembering that you can only receive it because Christ willfully accepted to be scourged, crowned with thorns, and crucified.
Please receive my humble prayers.