The best piece of advice I think anyone can possibly give someone fighting sins against purity is this: Turn to the Blessed Virgin Mary.
Our Holy Mother is the terror of demons, the terror of the devil. She is the most powerful intercessor against him and his minions, and against their snares.
Once a person falls into a vice, it can be very hard to get out of it. But there are steps we can take to show God we are truly sorry and that we are seriously fighting against it.
However hard or painful it may be, if someone in our lives is tempting us to sin, or if a group of people tempt us in that way, then we have a moral obligation to save our souls (and help save theirs) by separating ourselves from the occasion of our temptations: from those people, places or things that tempt us. This is a VERY important point! We cannot hope to escape temptations we are weak to, while continuing to expose ourselves to them. If we are seriously trying to be good, then we need to be honest with ourselves and with God, and take that fight seriously, even if it means great sacrifices for us, or others mocking and making fun of us. Even if it means giving up "friends" (who is a real friend to you, if they want you to go to hell with them?!), or "fun" (what fun will HELL be, that we would be willing to go there for a "good time"?), it is better to give up everything, than to go to hell.
So on the first place, you should ask yourself WHY, WHERE and with WHAT/WHOM you keep falling into these sins, and then begin to purge these things out of your life. It is one thing to be sorry that we have sinned. It is another part of what we have to do, to actually stop doing those things, or exposing ourselves to the dangers.
Secondly, as I said... Mary! Turn to our pure mother! She is called the "refuge of sinners" and "the gate of heaven" for a reason! She is the mother that will defend her children from dangers, and pray for them to be strong against temptations, especially when they ask her help. She is also the sweet gate or door of God's divine mercy for us lowly/big sinners, who need desperately that help, because we have become too weak, too miserable to save ourselves through a life of sin. And far from being only for big sinners, she is the mother of all sinners, the one ALL OF US are supposed to turn to and be devoted to.
I would recommend getting yourself a copy of the book "Consecration to Mary" by Fr. Helmuts Libietis. It contains all that is necessary to prepare yourself and to do the consecration of yourself to Mary. The devotion comes from St. Louis de Montfort, and reveals to souls the way of completely abandoning and consecrating ourselves, all that we are, all that we have and all that we do to Jesus in Mary, with complete and total faith in her, that far from leaving us without the graces of saving ourselves (since by it we give all our merits and graces to her), she will even more graciously and generously look after and help us, because we have kept nothing back for ourselves. The "holy slavery" to Jesus in Mary is really an excellent road for sinners, because by it we not only commit ourselves into Mary's hands and protection, but we sanctify our whole lives because then all of our actions, and all that we are and possess is for God.
The first part of the book may seem depressing, as it deals first with "cleaning house" so to speak... recognizing the sins and spirit of the world in ourselves, and then moving on to make a general confession (recommended for the devotion). But the latter 3/4ths of the book or so are full of the consolations that are to be found in this beautiful devotion of consecration and "holy slavery" to Mary.
One cannot possibly say enough about this devotion. If you have one foot in hell, Mary can save you. But you have to turn to her and ask her help, and this is really the best way of doing it. By making ourselves her "property" or rather Christ's property in her hands (consecrating ourselves), we recommend ourselves to her and her protection in a very special way. Then, if we sincerely try to live in the spirit of that consecration... turning away from sins and the spirit of evil and the world, toward God, through Mary... then our lives will be transformed in her, and more than likely we will, sooner or later, overcome our sins and vices, so long as we are striving against them sincerely, and by doing everything in our power.
Get yourself away from all of the persons, places and things that are tempting you, and run to Mary! Sincerely do everything in your power to get out of and away from these sins, and consecrate yourself to her, and become devoted to her especially, and she will help you win the fight against your sins and vices. She is the mediatrix of all graces... the channel of God's grace to men. Her divinely given "job" is to save as many sinners as she can. If you're honest with yourself and with God about really wanting to stop doing these things, then to avoid the near occasions of your sins, and turning to Mary is the best and surest way!