15 Next, in anticipation of future blessings, of which he speaks as already vouchsafed, he says, “We are satisfied with Your mercy in the morning”. Prophecy has thus been kindled for us, in the midst of these toils and sorrows of the night, like a lamp in the darkness, until day dawn, and the Day-star arise in our hearts. For blessed are the pure in heart, for they shall see God: then shall the righteous be filled with that blessing for which they hunger and thirst now, while, walking in faith, they are absent from the Lord. Hence are the words, “In Your presence is fullness of joy:” and, “Early in the morning they shall stand by, and shall look up:” and as other translators have said it, “We shall be satisfied with Your mercy in the morning;” then they shall be satisfied.
As he says elsewhere, “I shall be satisfied, when Your glory shall be revealed.” So it is said, “Lord, show us the Father, and it suffices us:” and our Lord Himself answers, “I will manifest Myself to Zion;” and until this promise is fulfilled, no blessing satisfies us, or ought to do so, lest our longings should be arrested in their course, when they ought to be increased until they gain their objects. “And we rejoiced and were glad all the days of our life.” Those days are days without end: they all exist together: it is thus they satisfy us: for they give not way to days succeeding: since there is nothing there which exists not yet because it has not reached us, or ceases to exist because it has passed; all are together: because there is one day only, which remains and passes not away: this is eternity itself.
These are the days respecting which it is written, “What man is he that lusts to live, and would fain see good days?” These days in another passage are styled years: where unto God it is said, “But You are the same, and Your years shall not fail:” for these are not years that are accounted for nothing, or days that perish like a shadow: but they are days which have a real existence, the number of which he who thus spoke, “Lord, let me know mine end” (that is, after reaching what term I shall remain unchanged, and have no further blessing to crave), “and the number of my days, what it is” (what is, not what is not): prayed to know.
He distinguishes them from the days of this life, of which he speaks as follows, “Behold, You have made my days as it were a span long,” which are not, because they stand not, remain not, but change in quick succession: nor is there a single hour in them in which our being is not such, but that one part of it has already passed, another is about to come, and none remains as it is. But those years and days, in which we too shall never fail, but evermore be refreshed, will never fail.
Let our souls long earnestly for those days, let them thirst ardently for them, that there we may be filled, be satisfied, and say what we now say in anticipation, “We have been satisfied,” etc. “We have been comforted again now, after the time that You have brought us low, and for the years wherein we have seen evil”.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)