Desgastante translates to "draining" or "exhausting." It's a descriptive word that goes beyond simple tiredness, implying a process of wearing down or eroding.
In the song, Ángela Aguilar uses it to describe the emotional toll of trying to rekindle a past relationship: "Qué desgastante intentar volver a ser los de antes" (How draining it is to try to be like we were before). This word perfectly captures the emotional exhaustion and futility of the situation, making it a powerful choice for the lyrics.
Ángela Aguilar spends this one doing the arithmetic of a breakup. The title line, "De este amor, nadie se va como llegó" ("From this love, nobody leaves the way they came"), is the thesis: both people walk out changed, and neither gets the old version of themselves back. She is blunt about who paid more. "Te di mi mejor versión, y tú a mí no" ("I gave you my best version, and you didn't give me yours") lands early, and the song then proves it twice over, because he leaves "aparentemente entero" ("apparently whole") and she is the one left going crazy.
The best image is the tattoo. She wants to scrub off "el tatuaje que hicieron tus besos" ("the tattoo that your kisses made"), which is a neat way of admitting the marks are permanent and she knows it. Then the song turns almost legal about the pain: "Lo más justo es repartirnos el dolor / En partes iguales, sin daños colaterales" ("The fairest thing is to split the pain between us / In equal parts, without collateral damage"). Asking an ex to settle heartbreak fairly is a bit much, and that is why it works. The ending refuses the comfort most ranchera ballads reach for: "Todo en la vida tiene solución / Pero tú y yo somos la excepción" ("Everything in life has a solution / But you and I are the exception").