None of us have the ability not to sin! The Bible teaches, first, that a superhuman spirit, the devil, is in rebellion against God and all he represents. The devil and his allies will do anything they can to get you to join their rebellion by rejecting God’s will for your life.
Second, there’s the world we inhabit. When the first people yielded to the devil’s temptation, they brought evil into the good world God had made. Now we live in an environment that is simply not going to encourage us to put God first, as he commands.
Finally, there’s our own nature. Like the world, we human beings are a good creation of God that went wrong very early in our history. Now there’s corruption within us that makes it impossible for us to offer God the perfect obedience that he demands and deserves. Our inner corruption means that the devil’s and the world’s temptations will strike a responsive chord within us, making it hard for us to say “no.”
Above all, please be sure that Jesus’ sacrifice has won God’s forgiveness for your every sin even the ones you can’t seem to shake. Second, know that Jesus also won for us the gift of the Holy Spirit, who creates a new self inside us. This new self wants to please God with a holy life. It’s locked in mortal combat with our old self, who wants exactly the opposite; and sometimes the old self still gets his way.
You might say there’s a dogfight going on inside every Christian. You could also say that the idea is to feed the dog you want to win, and starve the other one. Keep feeding your new self on God’s Word, and watch what happens!