I always wondered why I constantly failed as a Christian and then even as a Deist/Nominal Protestant and then as a Catholic. It never made sense to me if things were just as simple as: Satan and the Holy Spirit fighting it out for my soul. Certainly I believe that's part of it, but so many decisions seemed completely arbitrary or worse, completely up to me.
I've since studied Aquinas' view of the Fall (through that lovely website Called To Communion) and have entertained the notion that sin is a disease that goes against the natural order of your body and the subordination of the will to Reason. Thus all sin is basically failing to use reason to act in accordance with Virtue and the Spirit instead of simply gratifying desires.
All theories are just theories until you start applying them, and in my constant losing battle with lust and excess of all kind, I'm going to try to reason with myself when I enter temptation. Free Will is much bigger than I'd previously thought and it somehow survives the influence of concupiscence and of grace.
St. Augustine prays:
"O Lord, grant me the power to overcome sin. For this is what you gave to us when you granted us free choice of will. If I choose wrongly, then I shall be justly punished for it. Is that not true, my Lord, of whom I indebted for my temporal existence? Thank you, Lord, for granting me the power to will my self not to sin."
Obviously Scripture is infinitely above even holy Augustine, and so I hope to memorize this verse I've needed to make a way of life:
"If thou do well, shalt thou not receive? but if ill, shall not sin forthwith be present at the door? but the lust thereof shall be under thee, and thou shalt have dominion over it." - Genesis 4:7 Douay-Rheims
"If you do well, will you not be accepted? And if you do not do well, sin is couching at the door; its desire is for you, but you must master it." - Genesis 4:7 RSV
It was interesting looking at the Rheims translation here: http://www.latinvulgate.com/verse.aspx?t=0&b=1&c=4 because I could understand much of the Latin. The Vulgate says "et tu dominaberis illius" - lit. and you will have dominion of it (sin).
In one translation it's a moral imperative (RSV), and in the others it's a rhetorical question and promise / result clause. You will master it if you do well, if not it will be at your door, etc.
This I will have to combine with verses I found in studying the Sacrifice of the Mass, and the passage in 1 Pet 2:5 "offer spiritual sacrifices acceptable to God through Jesus Christ.". May my struggle against sin be a spiritual sacrifice acceptable to God through Jesus Christ's sacrifice.
... knowing me, I'm going to give in right away next time as I've created this big dramatic statement hah.