Well, being 18 years old, makes your biology a problem in itself. Your raging hormones are going to be a factor in having sexual desires. This is programed into us, which is how we are driven to mate. Confessors are also trained to understand the maturity of the individual in these matters.
That being said, there is sexual desire and there is lust, and lust is where we begin the desire to possess for self gratification. We possess the thoughts and if we could, we would possess another human being for our own gratification.
The first part, you will grow out of, trust me. I’m 61 and I recall how I was at 18, and I’m not even close to that anymore.
Another way is what I have been reading from the Eastern Orthodox Saints. St Ignatius the Syrian, advises to follow what our Lord Jesus Christ taught and that is, “to watch and pray.”
To watch Jesus is talking about being mindful of everything in our being, especially the thoughts which come into our consciousness. Watch these thoughts and when they are inappropriate, let them discard them, or let them dissolve. We can not stop thoughts from coming into our mind, and think is necessary, but we have so many useless and even evil thoughts, that we need to be mindful of those thoughts and become habitual in learning how to let go of the unwanted ones.
Next, when a thought that you are aware of that is unwanted, let it go and then pray.
The Eastern Orthodox are big fans of the Jesus Prayer, and I’ve become one to.
When the thought comes, mindfully let it go, and then recite the prayer, “Lord Jesus Christ Son of God, have mercy on me a sinner.”
It works because you are calling on the Lord for help, and he will never let anyone down who calls on his name for help in resisting sin.
Centered In Jesus Christ
Jim