“If we live in the Spirit, let us also walk in the Spirit.”  Galatians 5:25

In the preceding verse he reveals to us an awesome truth:
“And they that are Christ’s have crucified the flesh with the affections and lusts.”  Galatians 5:24
This means affections and lusts cannot have dominion over you, they are yours to control and channel in righteousness:
“For sin shall not have dominion over you…”  Romans 6:14
It simply means you can decide against whatever affections or lust you might have or feel, your soul- the seat of your emotions- is your property and you should exercise control.
“In your patience possess ye your souls.”  Luke 21:19
Your affections and lusts should be instruments of righteousness and it’s your job to ensure that happens.
“…but yield yourselves unto God, as those that are alive from the dead, and your members as instruments of righteousness unto God.”  Romans 6:13
In Christ, you have ceased to be a slave to your flesh, you are not under an obligation to live or plan your life according to the dictates of your flesh or the human nature:
“Consequently, brothers, we are not—with respect to human nature, that is—under an obligation to live according to human nature.”  Romans 8:12, ISV