This is a pretty common problem and condition.
You need to frame this sin as serious: (mentally, emotionally, and objectively) so as to retrain your mental rationale. Try this analogous thought:
I just killed someone so it doesn't matter if I go on killing, I'm already a killer so what's the difference?
Well, obviously the difference is the amount of dead bodies and lives lost so you are already telling yourself, I didn't kill anyone (your rationale continues to defend and deceive you).
but yes, you did . . . when you failed to respond to grace, you let the devil have another victory, a power which he uses to hurt somebody else. That is slowly causing death. You tip the scales in his favor. Sin is like his currency. Think about this seriously.
- Whose team do you want to play for?
Every time you give points to the other team, you hurt ours. It matters. God isn't the only one who sees your sin. The devil can too, and while he may not be able to read your mind like God can, he can see actions and figure out what is in the heart. When he has millions playing for his team, both knowingly and unknowingly, he uses that like capital ($$) before the Lord to advocate for the opportunity to seek further harm.
- Did you ever ask why satan had the audacity to demand the ability to, 31 . . . sift all of you like wheat, (cf. Luke 22:31)?
Jesus told Simon Peter than satan made that demand about the Apostles, but it is true for us too, and he couldn't make the case before God unless there was evidence to draw upon. He is like a shrewd prosecutor, and you will stand in his cross hairs, as he makes a case against you, if you simply let vice triumph over virtue.
Virtue is a long haul, one small victory at a time, over the course of months, maybe years. You will start to acquire virtue when you fight. Even secular motivational speakers will tell you it takes about seven weeks to create a new habit. That's like two months. That's the minimum. You need to get in the fight and set the target for the prize. Your temptation will likely never cease completely, but it will diminish in power and will become manageable. That is very doable.
Even the worst sexual addictions can be overcome in time. It's like peeling back an onion, one layer at a time, crying along the way.
- First, goes acting out on (porn, masturbation, fornication)
- then looking at other provocative material that triggers the problem
- retraining the eyes to look differently at the sex that attracts, be it same or opposite sex because these days homosexuality is a huge issue
- then finally changing the heart that looks at others through the eyes of Christ, with no lust whatsoever.
That final step may take a lifetime. Whatever area you struggle with, think about how someone in this situation must fight for freedom from this slavery.
Sin kills, grace restores. Go to Confession, pray the Rosary everyday, and go to daily Mass if you are struggling to break this sin habit. You must go to war on this if you hope to succeed.
I'm telling you, you can succeed if you partner with God, relying on His Strength but doing your due diligence. Whatever your game plan, (rationalization and rationalizing) is the enemy, and that is the worst habit of all to break.