In Psalm 21 He is on the other side of death and resurrection, and glorified. His heart's desire has been met with "the blessings of goodness" (verse 3).
In verses 8 to 13 the Messiah, Jesus is the Person addressed. In this Psalm is the Jewish Remnant's joy in the position of Christ with God: they had slighted Him when He was here, but now they understand His resurrection and ascension, and on Him their hopes are fixed.
The reader will note that there is no thought of the Christian hope here; not heaven, but earth is in view.