John Gill Commentary Psalms 89:39

"Thou hast abhorred the covenant of thy servant: Thou hast profaned his crown [by casting it] to the ground." — Psalms 89:39 (ASV)

You have made void the covenant of your servant
His servant David the Messiah (Psalms 89:3, 89:20). This means not the covenant of circumcision, nor the covenant at Sinai, which were really made void at the death of Christ; but the covenant of grace and redemption made with Christ, which it was promised should stand fast, and never be broken (Psalms 89:3, 89:28, 89:34).

It was thought to be null and void when the Redeemer was in the grave, and all hopes of redemption by him were gone (Luke 24:21). But so far was it from being so, that it was confirmed by the sufferings and death of Christ; and every blessing and promise of it were ratified by his blood, hence called the blood of the everlasting covenant (Hebrews 13:20).

you have profaned his crown by casting it to the ground:
by suffering it to be cast to the ground, and used contemptibly; as when Jesus was crowned with thorns, and saluted in a mock manner; when an "if" was put upon his being the King of Israel (Matthew 27:29, 27:42).

This seemed very inconsistent with the promise (Psalms 89:27) that he should be made higher than the kings of the earth. And yet so it was, and is; he is highly exalted, made Lord and Christ, crowned with glory and honour, and is set far above all principality and power, and every name that is named in this world or that to come, notwithstanding all the above usage of him.