A woman once admitted to me that she was obsessed with making her house clean. She was worried that she might miss one germ and someone in her family would get sick because of her failure. As she described it, it became clear to me that this obsession was becoming a fanaticism. I helped her through it, but it took a while.
One of the things that I told her was, "you will never find every germ". She responded with a bit of fear at first, but then I let her off the hook. I said, "and God knows that". God knows that we are merely living "dust" and that we are unable to succeed in anything without His help. God knows we want to be clean, and avoid sicknesses. He also knows where every single germ is (and knows them by name!), but He does not leave our health to us alone; He helps.
In the same way, I ask the question (especially to those who struggle with scrupulosity), "Can you find out all your sin?" And the answer is . . . No. You will never figure out every single sin you have committed. We are fallen people, with imperfect minds, and we forget things. If we were to believe that "just a little more" introspection will locate that one more sin we forgot, then we are becoming prideful. Certainly we can be lazy and fail to find out sin, but that is not what I am writing about. I am writing about when we are trying to do what is right and we still fall short. God does not leave our salvation to us alone (in spite of what non-catholics claim we believe!), He helps us.
God has told us (often) that He is sensitive to the fact that we are so weak. He knows how much we need Him and His mercy. We need His mercy not only because we sin and have to be forgiven, but also because we sin and miss it at times. We do it without thinking (and thus it is usually just venial) or we sin and then the challenges of life make us forget the sin in the midst of all the other things that flood our brains in an average day.
One of the proofs of God's goodness and love for us is the fact that He has given us the gift of purgatory. Imagine how few would make it to Heaven if we had to die in perfect grace. Yes, some do die that way, but only some. The Church teaches us that most need purgatory, precisely because they are unable to finish all of the temporal consequences of their sins. We do not do enough acts of penance. We miss things. We are weak and get distracted from our duties before the Lord. We need Him to help get us "cleaned up"; without the mercy of God we are totally lost.
Therefore, you will never find them all. You will miss something. You are not smart enough; you are not wise enough; you are not holy enough. That is why it is so amazingly wonderful to know God's patience with us. He is truly understanding of how difficult it is for us to obey Him. Yes, we still must work diligently to be faithful in all things; yes, we still must examine our conscience to see the sins we have committed and ask the Lord to reveal them to us so that we can confess them to a priest. And when our weakness keeps us from finding victory in everything, we can rejoice that God knows this and grants us extra grace when we confess to Him our weakness and failings.