When we have a choice to do things in a way that we know Christ would not have done then we are making a concious choice to sin. When we make these concious choices to sin we are turning or backs on Christ's teachings. Worse than that, we add to the suffering Christ bore for us on the cross. Christ died for all our sins, past present and future and non of us will be sinless, it is an impossible goal. But that does not allow us scope to sin when we feel like it, it does not give us carte blanc to do what we feel like when we feel like it.
We all have the capacity to sin, we are all human after all, but that also means we have the capacity, especially through Christ to choose not to sin also. Think about your life and think about the choices you make, your actions and your motivations. Are they all that they should be, or does sin enter in from time to time, is it that you are sometimes doing the right thing for the wrong reason. Even an action that looks right can be sinful if it is done with sinful and wrong motives.
If you want to live for Christ it requires a whole of life change, it is not enough to just say that you believe in Christ and want to live for Him. If that was true then you would be making the changes to live like Christ, you can't truly believe in the saving Grace and Love of Christ if you are content to remain the way that you were before you "found" Christ. Sure believing in Christ and the desire to live for Him is a good thing, but it is but one step of many steps to following Christ, taking one step towards Him is not following, following requires step after step after step, you have to walk. Just taking one step leaves the rest of your personal path with Christ unwalked and unexplored.
Each time we sin, each time we choose to go against Christ we are contributing to the suffering of our crucified Lord, we drive the nails further in we add to the weight on His heart and shoulders. When we knowingly and willingly choose to do what we know is wrong in Christ we become as much a part of the crucifixion as the centurion, Pilate or the Pharisees.
We know what Christ wants us to do, we also know our own human wants and desires, but we need to be mindful of where these desires come from and the price we pay for giving in to them. We need to ask God to help us with our weaknesses and to help us live without giving into them, but we also need to accept that help. We gain nothing from praying time and time again for help if when it comes along we choose not to accept it. Take a look at your life, look and see the places when you need it to be cleaned or even the parts that need to be cut away. Ask for help in doing this and then set to work with God's guidance.
Philippians 4:13 (KJV)
I can do all things through Christ which strengthens me.