Inyectarme literally means "to inject me." It's a strong and somewhat unexpected verb to find in a song about a past relationship.
In "El Pasado," Zacarias Ferreira uses this word metaphorically to describe how the past tries to infuse him with its "bad love" and negativity. He resists, saying "el pasado fallo queriendo inyectarme de su mal amor pero yo no lo deje proceder" (the past failed trying to inject me with its bad love, but I didn't let it proceed). This vivid imagery makes the word memorable and impactful.
Zacarías Ferreira's "El Pasado" is a powerful declaration of independence from past mistakes and heartbreaks. It's like a conversation with a persistent ex, but that ex is actually the singer's own past! He's telling his past, "Guarda tu veneno porque ya no soy ese niño" [Keep your venom because I am no longer that child]. He's grown up and isn't going to fall for the same old tricks.
He makes it clear that his past has nothing new to offer him, saying, "si el pasado vuelve a buscarme vuelvo y me rebelo y no le abro la puerta porque nada nuevo él tiene a mí que ofrecerme" [if the past comes looking for me again, I rebel again and don't open the door because it has nothing new to offer me]. He even mentions a moment of weakness, admitting, "Por un segundo creí que mi amor la trataría mejor" [For a second I believed that my love would treat her better], but quickly corrects himself. He's done with the "falsas y todas tus mentiras" [falsehoods and all your lies] and tells his past to get on a train "con destino a no volver a fallar" [with a destination of not failing again]. It's a really strong message about moving forward and not letting old hurts define you.