Sumerjo is the 'I' form of the verb sumergir, which means 'to submerge' or 'to plunge'. It's a powerful verb that creates a vivid image of being underwater.
In this emotional ballad, Laura Pausini sings "Y las sumerjo en un lamento" (And I submerge them in a lament). She uses this word metaphorically to describe drowning her unspoken words in sadness, words she can no longer say to the person she misses.
This song is a heart-wrenching ballad about regret and loss. Laura Pausini sings from the perspective of someone who has lost a person they love and is now haunted by all the things left unsaid. She's filled with words she can no longer share, describing them as "Las cosas importantes / Esas palabras / Que nunca escucharás" (The important things / Those words / That you will never hear). The feeling is so physical that she says they "se rompen en mis dientes" (break in my teeth).
The central phrase, "En cambio no" (Instead, no), acts as a constant, painful return to reality. She wishes she could talk to this person, explain things, or even just ask if she loved them enough, but it's too late. The song suggests the loss is permanent and sudden, leaving her with a thousand things to say but no one to say them to. She remembers happier times, like when they were "corríamos al viento" (running in the wind), which only makes the present feel more empty now that "Diciembre ya llegó / No estás aquí" (December has arrived / You are not here).