Paul said that before he understood God's law he felt happy, free and alive. But when he read that he was a sinner, he found God's law condemning him to death.
Paul has been describing the effect which God's law had had on him. It had made him feel guilty for his sin and strangely inflamed his natural desire to do what displeases God.
The Apostle Paul confesses that he is not in full control of himself. For an educated man with theological training he was amazed at his inability to regulate his own behaviour.
The Christian life is a struggle. It was for the Apostle Paul and it will be for us. Our hearts know what is right, and we want to obey God, but there is always something inside us saying 'No'.
Despite our sinfulness, our inability to control or reform ourselves, and despite the ineffectiveness of religious rules ... those who believe in Jesus are released from condemnation.
We behave according to our desires. If we want it, we will think about it, and plan how to get it ... then we take action. So it matters greatly how we manage our desires and our thoughts.