Psalm 110: 1: A Psalm of David. The LORD said unto my Lord, Sit thou at my right hand, until I make thine enemies thy footstool.
The “LORD” of Psalm 110: 1 is Jehovah God.
The “Lord” of Psalm 110: 1 is in Hebrew, “adonai” (king, lord or master), and represents a man.
Genesis 1: 1: In the beginning God “Elohim” created the heaven and the earth.
Exodus 21: 6: Then his master shall bring him unto the judges “elohim”; human judges; he shall also bring him to the door, or unto the door post; and his master shall bore his ear through with an aul; and he shall serve him for ever.
The “Elohim” of Genesis 1: 1 is Almighty God; the “elohim” of Exodus 21: 6 speaks of human judges.
“Adonai” (the second lord of Psalm 110: 1) is used many times in the Old Testament.
Genesis 23: 6: Hear us, my lord “adonai”: thou art a mighty prince among us: in the choice of our sepulchres bury thy dead; none of us shall withhold from thee his sepulchre, but that thou mayest bury thy dead.
Genesis 24: 36: And Sarah my master’s “adonai” wife bare a son to my master when she was old: and unto him hath he given all that he hath.
Exodus 21: 5: And if the servant shall plainly say, I love my master “adonai”, my wife, and my children; I will not go out free:
Psalm 110 is a psalm written in the third person by David about himself.
Psalm 110 (and many other psalms) was written in the third person because it was to be sung by the Levites in the temple; they would sing of David as their lord (“adonai”).
Psalm 110 is the Levites singing of how the LORD (“Jehovah”) gave David (“adonai”) an assurance of protection, to make David’s enemies (the Philistines) a footstool, for they would be brought into subjection.