Many ancient and modern writers make this Psalm a part of the former. They have failed to see that the strain is now more gladsome and hopeful.
The hart is now bounding on to the water brooks. The psalmist is claiming his right to refreshment, and anticipating it as at his very lips.
The gloom of night (42:8) and of mourning (v2) are to be exchanged for favour or light (v3) and truth, i.e. the fulfillment of the promises made to him (Aben-Ezra) shall soon show that he has not been forgotten (42:9); and soon God shall be his jubilee song "joy of his joy" and the harp shall celebrate the well pleased face of "Eloi, Eloi", my God who once seemed to stand far off.
To Christ and to his members, the highest gladness (spoken in v4) comes from The Altar with its accepted sacrifice. Christ risen, and Christ ascended, are pointed out in this; and it is in his resurrection and ascension that we see the sacrifice accepted, and our hearts learn true joy.
No doubt this same source of joy is to be opened up to us more fully still when He appears the second time "without sin" for salvation, and all enemies are put under him.
He, too, shall rejoice afresh in that day, drinking from the coolest of the longed for water brooks. Let us, meanwhile, read and sing this Psalm in happy confidence, as The Righteous One's claiming his right to full refreshment.