The Spirit of God, who raised Jesus from the dead, lives in you. And just as God raised Christ Jesus from the dead, he will give life to your mortal bodies by this same Spirit living within you.
The sinful nature wants to do evil, which is just the opposite of what the Spirit wants. And the Spirit gives us desires that are the opposite of what the sinful nature desires. These two forces are constantly fighting each other, so you are not free to carry out your good intentions.