The focal point of this psalmist, a fella who has his back against the wall and is surrounded by those who want to do him in, is this:
The sum of your word is truth;
and every one of your righteous ordinances endures forever.
It is no wonder that the rabbis love this psalm; it is Torah-centric.
The “sum” or other ideas connected to this term (rosh): tip of a mountain, top of the tower, top of the mast or tree, the ears of a corn, the chief of a nation or group, the best… etc..

The whole is put together into this one word: God’s Word is Truth; everything God says is eternally true. God can do no other.
For that reason, the psalmist faces God with his back against the wall, pleas with God, and asks God to preserve his life.