There are a lot of techniques that can be used for this particular sin and it sounds like you’re on the right path with some of them.
Prayer, rosary, scapular, frequent confession – those are excellent. Mass and communion is a powerful remedy also.
After that, you should keep the thought of God in your mind. Don’t hesitate to affirm to yourself that you are a good person, in the grace of God. You want to be a gentleman and God wants you to be successful in His plan for you.
You can never remember enough that God loves you and His mercy is endless for you. He will wash away all your sins and make you pure and a true man for Him (and for your wife).
All of that said, there are some things that many do not want to accept. The first is that this can be a very addictive vice. This is what I think you’re suffering from.
Think of it like trying to quit smoking cigarettes – only much harder. I hate to say it like that, but it can be compulsive.
It’s a tricky sin to handle. Temptations will come upon you without warning and they can be strong. You have to be prepared with grace and strength and a strong mental formation. You become prepared by your daily effort.
I’ll mention just a couple more remedies, and there are others.
The first is aversion therapy. This is something you do yourself, you don’t need a therapist for it. Basically, you are taking pleasure and comfort for yourself – taking them from what is reserved only for marriage. So, a good way to stop is to give yourself pain each time you have a lustful thought or if you fall into that sin. Give yourself a sharp pain of some kind (not going to extremes). You will then know that this sin carries a price in pain.
Cigarette smokers do this. They make the cigarette taste bad and then they don’t want to smoke them.
For you, it’s doing something that hurts (basic idea of penance) every time you fall into this. It’s best when you just get a feeling for this sin – like you’re drifting towards it.
You also have to investigate what triggers this kind of sin. Is it the TV shows you watch or music you listen to? Eventually, you may find that something pushes you nearer to sin – in that case, get rid of it from your life.
The final remedy is the one St. Paul recommended – rather than falling into sin all the time, it’s best to get married. This will provide it’s own challenges with chastity, but you’ll have the sacrament of marriage to help you.
Unfortunately, in our day and age, nobody will recommend that a 17 year old should get married. Thus, people wait a long time and they have a lot of temptations to deal with.
I don’t know what to say on that really, except I don’t think getting married at 18 or 19 is such a horrible thing if both parties are mature and truly are friends (as you mention) and have the Catholic Faith in them.
But in any case, this sin is extremely common and very few people really work on overcoming it. Your efforts will be blessed – you’ll gain courage, wisdom and strength in the battle.
No matter what, don’t give up. If you fall, take it to confession.
You might also try a spiritual director to meet with – that can be one of the best things to do. Someone to talk with about this problem can help to bring it out of a secret sin and something that you can work on each day and each week to overcome.