Part of the speaker's struggle to get back to the past has to do with the fact that he is struggling with his own mortality; he is getting old, facing death, and trying to remember his youth. "Piedra de sol" is filled with images of death and dying, some of them poetic, and others kind of gruesome. But if you read to the end (we know, it's a long way), you'll see that mortality gets turned upside-down and becomes a birth. It's a nice thought after all that doom and gloom.
In "Piedra de sol," death is presented as an inevitable and horrible part of life.
The speaker in "Piedra de sol" embraces death by finding a way to become both everyone and no one at the same time.