Sufriendo is the present participle of the verb sufrir (to suffer). It's a highly emotional word that conveys deep pain and anguish.
In this heartbreaking ballad, Selena uses it to express the intense torment of unrequited love. The line "Quizás me muera sufriendo" (Maybe I'll die suffering) is incredibly dramatic, perfectly capturing the feeling of desperation for a love that will never be returned.
This song is a classic tale of loving someone who doesn't love you back. Selena sings about being completely consumed with thoughts of a person, admitting, "Siempre voy pensando, pensando en ti" (I'm always thinking, thinking of you). This brings her a mix of joy and sadness, because she knows it's a one-sided feeling.
She calls herself "triste y tonta" (sad and silly) for even imagining that this person would look her way. It's a feeling of hopeless devotion, where she knows her dreams are just "sueños locos, pura ilusión" (crazy dreams, pure illusion). The song's central, heartbreaking idea is that this person will never love her "Como te quiero yo a ti" (The way that I love you).