Galatians 6:8

He who sows to his flesh will of the flesh reap corruption, but he who sows to the Spirit will of the Spirit reap everlasting life.  

The flesh is a battleground all by itself.  That is why Paul said he kept the flesh under control.  Our spirit was born again and renewed while our flesh is constant work.  The stronger we make our spirit, the easier it will be to control the flesh.  It takes time and constant practice.