“Trust in the LORD and do good; 
Dwell in the land and cultivate faithfulness. 
Delight yourself in the LORD; 
And He will give you the desires of your heart.”
‭‭Psalms‬ ‭37:3-4 NASB‬‬
The margin of my bible tells me that the phrase, "cultivate faithfulness," can also be translated as "feed on His faithfulness." In the context of this passage (living among evildoers, tempted to fret over and be envious of them), this makes all the difference!
Despite dwelling in the land (ie among evildoers), yet we are called to feed on God's faithfulness. Meaning, the "food" which we consume is not supposed to be the things we see and hear in the land, among the wicked, but rather we are meant to be sustained by the spiritual reality of God's goodness and faithfulness. 
Another Psalm comes to mind:
“It is good to give thanks to the LORD 
And to sing praises to Your name, O Most High; 
To declare Your lovingkindness in the morning 
And Your faithfulness by night,”
‭‭Psalms‬ ‭92:1-2‬ ‭NASB‬‬
One reason that it is so important ("It is good...") to declare His love in the morning and His faithfulness at night is that we need to constantly remind ourselves of Who God is, so that we might revel in and feed upon His faithfulness, which He intends to be our sustenance while we are dwelling in the land. As we declare His love and faithfulness, that same love and faithfulness floods our hearts. The practical outcome is that we become truly delighted in the Lord (37:4). Proactively seeking to be reminded of God's lovingkindness is critically important to our spiritual health. 
“I have set the LORD continually before me; 
Because He is at my right hand, I will not be shaken."
Psalms‬ ‭16:8 NASB
What diligence we have for an example here. Setting the Lord before me must never be reduced to a daily activity. I shouldn't ever check the box on feeding on God's faithfulness, because I don't ever get "done with" setting Him before me. It must be a continuous attitude of my heart.  
The phrase "before me" means more than just "in my line of sight," like a toy we would put on the dash of our car; more than that, it means putting God first. David cultivated the habit of putting the Lord ahead of himself, choosing to elevate God's interests above his own. Just like letting someone go through a door ahead of him, ("no really, you go first,") David's attitude was, "no really, Your will be done, Lord." We see this all throughout his life, especially as he was seeking refuge from evil King Saul. He was conscientious to put God's agenda first, and what he received was so much greater: the conscious awareness that God Himself was by his side!
Feeding of God's faithfulness is what allows us to live in this world with peace and rest, because He uses those moments to remind us Who He is, which gives us confidence to face our lives "dwelling in the land."
Psalm 16 goes on to say:
"You will make known to me the path of life; 
In Your presence is fullness of joy; 
In Your right hand there are pleasures forever.”
‭‭Psalms‬ ‭16:11‬ ‭NASB‬‬
He who sets the Lord before himself -- who actively takes pains to set aside distractions, affections, etc, and gives the Lord the place of attention and honor -- is the one who can confidently claim God's direction and joy and rest. 
If we feel at any time that any of these are lacking, then let us first go backwards in the Psalm and seek to set the Lord continually before us. It is to such people that God gives entry into His direction, joy, and rest. 
What a blessing it is to feed on God's faithfulness!