The OP offers us two choices: either Daniel mourned because of what he had already been shown, or what he was about to be shown. It does not make sense that he would mourn for something he had not yet been shown, so of the two choices, the first is better. In any case I suggest three reasons for his mourning.
Because he could not return with the other Exiles
Note the timing of mourning:
In the third year of Cyrus king of Persia a message was revealed to
Daniel, whose name was called Belteshazzar. The message was true, but
the appointed time was long; and he understood the message, and had
understanding of the vision. In those days I, Daniel, was mourning
three full weeks.
Cyrus was the king who enabled the Jews to return to Jerusalem and rebuild the Temple. The first wave of exiles had already returned by the time in question. Daniel could not be among them, apparently because of his advanced age. To return and rebuild the Temple was the great hope of the Jews at that time, promised to them by both Ezekiel and Isaiah. Cyrus had now made this possible, but Daniel, who lived most of his life in exile, could not participate. So his inability to join them was probably one reason for his mourning.
Because he knew his people would suffer
Another reason would indeed be his foreknowledge that his people would suffer. (As mentioned he would not mourn because of what he was about to be shown.) Although the timing of his visions is confusing we can presume that he knew his people would suffer.
He mourned in repentance
However, since a type of fasting was involved, we should also consider that Daniel mourned for his people's sins. The theme of national repentance looms over this entire period, not only in the Book of Daniel but also in Isaiah, Jeremiah and Ezekiel. Chapter 9 of Daniel expresses this theme eloquently.
I turned my face to the Lord God, seeking him by prayer and
supplications with fasting and sackcloth and ashes. I prayed to the
Lord my God and made confession, saying, “O Lord, the great and
terrible God, who keeps covenant and steadfast love with those who
love him and keep his commandments, we have sinned and done wrong
and acted wickedly and rebelled, turning aside from thy commandments
and ordinances; 6 we have not listened to thy servants the prophets,
who spoke in thy name to our kings, our princes, and our fathers, and
to all the people of the land. (3-6)
The above prayer represents the attitude of all the major prophets and pious Jews of the time. Ultimately, it is the best way of understanding why Daniel mourned.
Addendum "Darius the Mede" is unknown to history outside of the Book of Daniel. A note provided by the US Council of Catholic Bishops says "The Median kingdom did not exist at this time because it had already been conquered by Cyrus the Persian. Apparently the author of Daniel is following an apocalyptic view of history, linked to prophecy (cf. Is 13:17–19; Jer 51:11, 28–30), according to which the Medes formed the second of four world kingdoms preceding the messianic times... The character of Darius the Mede has probably been modeled on that of the Persian king Darius the Great (522–486 B.C.), the second successor of Cyrus."