For to be carnally minded is death, but to be spiritually minded is life and peace."
Romans 8:5-6 (NKJV)
What if you moved into a new house or apartment, and even shipped all of your belongings to your new place, but you continued to try and live in your old house or apartment? It could be that your new home was not too far from the old one so you could go back and forth trying to live in two places, but that would not make any sense. With all of the things you need to live your life located in your new home it would be foolish to try and keep living in your old place, but you could actually still try to do it.
In that same way that you cannot really live in two places at once physically, the same thing is true spiritually. We have a choice as to what we identify as our home, naturally and spiritually. Spiritually speaking, we are a new creation in Christ (2 Corinthians 5:17) seated with Christ in heavenly places (Ephesians 2:6). If we focus and base our life on natural things it will keep us living the same way we always did. As we focus and base our life on spiritual things this helps us to live the way that God has designed for us to live.
Spiritually or naturally, you choose what to focus and base your life on, but you cannot live in two places, your head and your heart, at the same time. This is what Paul was talking about in Romans 8. It is our choice as to where and how we will live, in the natural or the spiritual. What we focus our life on will determine whether we partake of the spiritual blessings that belong to us, or if we will continue to live as the same type of person we were before we became born again.