If there is a sin you are having a hard time overcoming, the best thing to do is to go to Confession frequently and confess it. The Sacrament of Reconciliation gives us specific graces to overcome the sins we confess.
And just knowing that you will have to keep confessing the same sin over and over again may help you avoid it.
I would recommend going to confession every week if there is a serious sin you are trying to overcome.
Praying the Rosary every day, asking Mary to pray for you for the virtue of purity should help too.
