Tragarme literally means "to swallow myself" or "to swallow something whole." While it can refer to physically swallowing, in this song, it's used metaphorically, which makes it particularly interesting.
The lyric "Tener que tragarme un 'te quiero'" translates to "Having to swallow an 'I love you'." This powerfully conveys the feeling of holding back deep emotions and unspoken words, a common experience in unrequited love. It's a vivid and relatable image of suppressing one's true feelings.
Morat and Camilo built this whole song around one repeated question, "¿Por qué me toca a mí?" (Why does it fall to me?). The guy singing it is watching someone he wants be with somebody else, and he says it plainly right at the start: "Tú estás con alguien más y yo contando mis dudas" (You're with someone else and I'm counting my doubts).
What makes the song work is the friend-zone detail. He isn't just hurting, he's performing being fine, "Fingir que estoy hecho de acero" (Pretending I'm made of steel), while living "disfrazado de amigo" (disguised as a friend) and wanting everything with her. By the second verse he's already decided he'll act, "voy a dar el primer paso" (I'm going to take the first step), and he even offers to pick up the pieces if her heart ends up broken, "yo recojo los pedazos" (I pick up the pieces). Then the hook just keeps asking the same question, which is exactly what it feels like to be stuck this way.