Amaneció is the past tense of the verb amanecer, which literally means "to dawn," like the sun rising to start a new day. It’s a beautiful word on its own, but its use in this song is what makes it truly special.
Selena Gomez uses it metaphorically, singing, "Tristeza en mí amaneció" (Sadness dawned in me). Instead of just saying she became sad, she paints a picture of sadness arriving with the same finality as a new morning, marking the beginning of a new emotional state. This poetic twist makes the feeling of heartbreak much more profound.
This song tells the story of a very difficult goodbye. Selena Gomez starts by singing to someone she is leaving, trying to comfort them even though she's the one ending things. She sees their pain, singing, "Those sad eyes, sad eyes / You know I don't wanna say goodbye." The song then switches to Spanish to describe this person as "El muchacho de los ojos tristes" (The boy with the sad eyes), who she believed she could make happy with her love.
But then, the story jumps forward. A new verse reveals, "Han pasado dos años" (Two years have passed), and everything has changed. The roles are now reversed. She remembers how it used to be his "ojos lloviendo en otoño" (eyes raining in autumn), but now she says, "ahora son los míos" (now they are mine). The person who was trying to fix someone else's sadness has ended up heartbroken herself.