There is this children's book called "If You Give a Mouse a Cookie..." I think I may have read it a thousand times (to my children, not for my own reading pleasure). It has to do with cause and effect (yes, I do critically examine children's books) and how one action leads to another. The beginning goes like this: "If you give a mouse a cookie, he's gonna want a glass of milk to go with it..." (and then proceeds to show the line of events that follow). In other words, if you open yourself up there is potential for a domino effect.
With that principle in mind, let me take it a bit further. "If you give the devil an opportunity, he's going to want your soul to go with it..." That "opportunity" can be seen as a chance, a compromise, a wink and nod (etc.). All too often people (including devout Catholics) think that they will be fine with just a "little" sin. They do not consider that one sin leads to another. They forget that many little venial sins can soften your heart to corruption and enable you to give in to a grave sin that you would have never thought possible a short time before.
What are your big sins that you struggle with? I can guarantee that they came from something smaller. At some point in time you "gave the devil an opportunity" and he took it. Then he started working on you--bit by bit, piece by piece--and eventually you gave in to the bigger sin. Watch, therefore, what you are softening on. Pay attention to the "little" things that do not appear to be a big deal, because they can turn into a big deal in a short amount of time.