Desolador translates to "devastating" or "bleak." It's a strong and evocative adjective that isn't as common as other words for sadness.
In the song, Abraham Mateo uses it to describe a look: "Cómo olvidar ese mirar desolador" (How to forget that devastating gaze). It paints a vivid picture of the profound sadness and despair in someone's eyes, making it a memorable and impactful word in the context of heartbreak.
"Without You" is a breakup song told after the fact, from the side that stayed quiet. When his partner said she was leaving, he watched her smile even while her eyes gave her away: "Mas tus ojos eran trozos del dolor" ("But your eyes were pieces of pain"). He chose silence over a fight, "No quise hablar" ("I didn't want to talk"), and only managed one word at the door: goodbye. He's been regretting it since.
The chorus is where the song actually lives. "No sé vivir si no es contigo" ("I don't know how to live if it's not with you") repeats until it stops sounding like a lyric and starts sounding like someone talking themselves in circles. Abraham Mateo and Caroline Costa trade the verses back and forth, so the song plays like an argument five years too late, the kind where you finally say everything you should have said with nobody left to hear it.