Who knows ancient poetry? Can you tell me what this means? Thank you very much

Generally speaking, it's just that one person has changed a lot and is alone. In the first sentence, he wrote the scenery in front of him, played up the atmosphere, and thought of the gentleman behind him, that is, what he saw and thought of that person. In the second sentence, he still writes the scenery first and the feelings later. In this case, it is difficult to control his emotions. The third sentence, yesterday's joy is gone now, just a memory, leaving me alone, alone on the road.