The grammar is strange here, too. Praecipue is an adverb but seems to be largely referring to dolor, which in turn is acting in a vaguely adjectival manner. The overarching sense of the full phrase is something close to “this was especially painful for me,” but there is no grammatical this, and no verb to describe its action or its being. There’s a hole in the center of the line, not unlike the chiasmatic skip that sometimes describes the indescribable in the elegists: something happened to turn Gallus from living neighbor into cast-out bones, but the narrator either cannot or will not get at quite what that something was.
It is only from the dead that Aeneas learns the shape of his fate, after all: the dead and the yet-to-be-dead, who were and are and are to come. Dead men tell all sorts of tales, in Roman literature, and those tales tend to be about things that brush up against the architecture of Fate. Who can tell the story of the Odyssey, except for the dead Odysseus himself?