I am working to restore and old motorcycle right now. Part of the restoration process involves removing rust and there are various techniques for this, but all require a bit of elbow grease. The more rust, the more energy is needed to achieve success. The same is true for our souls. The more sins (even just little venial ones) that build up in our behavior, the more it takes to overcome them because we become callous to them very quickly. As the Catechism tells us, a number of venial sins can be worse than one grave sin because we become lax about them and forget we have committed them. Thus, the effort to overcome a group of venial sins can actually be much worse than we realize if our hearts have gotten used to them. The "rust" builds up and becomes worse and worse over time. This is why the Church requires yearly confession as a minimum -- to help us pay attention.
How much do you need to deal with? How hard will you need to work to overcome? How long is it since you have been to confession?