[stone of vision]

In Frodo's departure, there is the sorrow of the end of an ancient era, the incertitude of a new one.
A transition between a world doomed to vanish, evolving to a brand new world doomed to be forgetful of what had existed in order to grow up for its own.
The time of Frodo was in the past story, not fitting anymore in the young world born from the ashes of the ancient Age.
Gaining wisdom, Frodo is able to sense as the Elves that his/their "time" in "this" Middle Earth is over.
In the other hand, sailing to Grey Haven might also be a sort of re-birth for him, a return to the source of light, his new knowledge would need to improve and satisfy.
A birth is always a moment of great satisfaction and pain, and the separation of Frodo and Sam, Frodo and Middle earth is painful, and nostalgic because it is his farewell to the last links of his "preview " life.
The new Age of Middle Earth needs to move on of what was- and Frodo was one of them- , as much as Frodo needs to leave it, to evolve both.
The people who contribute to a new and better world are not unfortunatly the ones who could live and blossom in it.

Sad but hopeless.