Romans 8:12-13

We have an obligation, but not to live according to the flesh. This can be taken in two ways, both of which are fruitful:

1. We do not have to do what the flesh tells us to do. Our flesh makes demands upon us, like a spoilt child, demanding that it get what it wants. We are not obliged to do what it says. Our flesh can demand that we lose our temper or give in to resentment. It tells us that we absolutely have to worry about something. The truth is that we do not actually have to give in to it. In fact if we resits the demands of the flesh they will eventually subside and go away.
2. Another way of looking at what Paul says is this. He is presenting living by the Spirit as the antithesis of living by law, of seeking to be righteous by our own efforts. We know that we should be good, and this is right, but then our natural reaction is to seek to be good by our own strength. We can even feel that we are obliged to do this, feeling guilty if we do not. I.e. we feel a moral obligation to seek to do the right thing by our own strength and effort.

The truth is that the obligation we have is to live by the Spirit. For if we live by the flesh we will die. This is true whichever of the two alternatives above you take. If we keep giving in to the demands of the flesh, then it leads only to death. Similarly, if we seek to be right out of our own strength this will inevitably result in failure.

Instead we are to put to death the "misdeeds of the body" by the Spirit. The misdeeds of the body are any actions, thoughts, attitudes that are contrary to God's will. This includes overtly sinful stuff like lust, lying, and things like living in fear or anxiety.

How do we put these "misdeeds" to death by the Spirit? We listen to God and do what He says. Let me give you an example. All of us are prone to anxiety at some points in our lives, some more than others. The anxiety demands that we pay attention to it. But Jesus tells us that we do not need to worry (Matt 6:25-34). In Philippians 4:6-7 Paul tells not to worry about anything. That "anything" includes the vary thing we are tempted to worry about right now. When we are tempted to worry we should instead spend the time praying for something more useful. Do not pray about the thing you are worrying about, but pray for your family, your church leaders, your neighbours, your city or nation. At first you might not feel much different, but over time (might be days, or even weeks) you will find that as Phil 4:7 says, God is guarding your heart and mind, and you are not worrying anymore. Likewise, if we feel angry against someone, pray for God to bless them, pray for their good, for Jesus tells us to love everyone, even our enemies. At first we may find that we have to do this through gritted teeth, not feeling very loving at all. Again, over time we will find that we start to change. For through the Spirit we are putting the misdeeds of the body to death. When we do that we will live a far richer life.