Everyone has had that feeling of helplessness. That feeling when it seems we just are not able to do the right thing or to make the right choice. To use Paul's language those moments when our sinful nature is what controls our actions and thoughts. If Paul would have stopped writing at that point our lives would in fact seem hopeless.
However, Paul does not stop writing there. The account in Romans continues to remind us that we are no longer slaves to our sinful nature when the Spirit of Christ is within us. More directly stated when we are followers of Jesus we are no longer helpless to the power of sin. To be clear this does not mean we are no longer pursued by sin. Instead when we feel the power of our sinful nature lurking, the Spirit of God is ready, willing and able to carry the day.
Volition is a big part of this. First we must choose to follow Christ. Next we must choose to make ourselves available to the leadership of Christ in our lives. In more churchy terms this is have in Jesus be the forgiver and leader of our lives. Once we make those choices we have available to us great resources to withstand the prowling sin of this world.
This is also true of a marriage. To experience the fullness of marriage we have to make the choice for Christ to be our forgiver and leader as individuals and as a couple. It is amazing the number of people who navigate life and marriage without the grace and guidance of the Spirit of God found in Christ Jesus.
Some might argue that you do not need to have Christ as part of the equation to experience the fullness of life or a marriage. While it is entirely possible to experience a good life, maybe even a great life outside of Christ, it is not possible to experience the fullness of life outside of Christ. This also means the fullness of marriage, and that fullness is found when we allow Jesus to be both forgiver and leader of our marriage.