His seed shall endure forever ... - His posterity. See the notes at Psalm 89:29. There, the expression is, "his throne as the days of heaven." Here it is, "his throne as the sun before me." The meaning is the same. It would stand through all time. Compare the notes at Psalm 72:5.

Psalm 89:36 Parallel Commentaries

A vision of the King.
ONE of the most blessed occupations for the believer is the prayerful searching of God's holy Word to discover there new glories and fresh beauties of Him, who is altogether lovely. Shall we ever find out all which the written Word reveals of Himself and His worthiness? This wonderful theme can never be exhausted. The heart which is devoted to Him and longs through the presence and indwelling of the Holy Spirit to be closer to the Lord, to hear and know more of Himself, will always find something …
Arno Gaebelein—The Lord of Glory

His Future Work
The Lord Jesus Christ, who finished the work on earth the Father gave Him to do, who is now bodily present in the highest heaven, occupying the Father's throne and exercising His priesthood in behalf of His people, is also King. To Him belongeth a Kingdom and a kingly Glory. He has therefore a kingly work to do. While His past work was foretold by the Spirit of God and His priestly work foreshadowed in the Old Testament, His work as King and His glorious Kingdom to come are likewise the subjects …
A. C. Gaebelein—The Work Of Christ

Assurance
Q-xxxvi: WHAT ARE THE BENEFITS WHICH FLOW FROM SANCTIFICATION? A: Assurance of God's love, peace of conscience, joy in the Holy Ghost, increase of grace, and perseverance therein to the end. The first benefit flowing from sanctification is assurance of God's love. 'Give diligence to make your calling and election sure.' 2 Pet 1:10. Sanctification is the seed, assurance is the flower which grows out of it: assurance is a consequent of sanctification. The saints of old had it. We know that we know …
Thomas Watson—A Body of Divinity