Every promise of God has a condition that must be met before it can become "yours". The condition for salvation (John 3:16) is belief.

The condition for Psalm 91 to be "yours" is to dwell in the shelter of the Most High.

The condition to know the truth and have the truth set you free is: If you continue in Christ's word. John 8: 31-32.

I could go on for hours (years, actually) but the point is to ALWAYS look for the condition before you "own" God's word.