Matisse opens this one with the sweetest domestic scene. "Te hice el desayuno a las 7 AM en punto" (I made you breakfast at 7 AM sharp), music on in the house, coffee poured into her cup. Then he tells you his favourite hour is the end of the day, when nobody interrupts and they talk all night. "Todo es perfecto aquí" (Everything is perfect here). It sounds like a love song.
Then the floor drops out: "Pero no estabas, solo me lo imaginé" (But you weren't there, I only imagined it). The whole morning was in his head. The title line explains why: "Es mi mecanismo de defensa" (It's my defense mechanism). Pretending she is still around is how he copes, because he cannot get used to missing her and "vivo solo en el ayer" (I only live in yesterday).
The imagery after that gets bleak in a quiet way. He is left standing in the middle of the street ("parado a media calle") wondering how many hours have passed, and life slips away "como arena entre mis manos" (like sand between my hands). A nice phrase for learners: "me hagas falta" comes from hacer falta, the everyday Spanish way to say you miss someone, literally that they are lacking to you. And "platicamos" is the Mexican favourite for chatting, where Spain would usually say charlamos.