Tropezabas comes from the verb tropezar, meaning "to stumble" or "to trip". It's used here in the imperfect tense, indicating a past habitual action.
In the song, the lyric "Y tropezabas muy pocas veces" (And you used to stumble very few times) paints a picture of a past self that was perhaps more graceful or less prone to mistakes before the narrator's influence. It's an interesting word because it evokes a physical action but also hints at metaphorical stumbles in life, making it both vivid and relatable.
This is a love song that never describes the love. It describes what the love replaced. Silvana Estrada builds the whole chorus out of broken household things: "Una taza con el mango a punto de caer" (a cup with the handle about to fall off), "Un cajón que no cierra, un mechero mojado" (a drawer that won't close, a wet lighter), "Un aplauso mal dado en medio de la función" (a badly timed clap in the middle of the show). Each one still works, just barely, and each one is slightly wrong in a way nobody would bother fixing. That is the self-portrait. Then she gives it away anyway — "Yo que no tenía nada, igual te lo di" (I who had nothing, still gave it to you) — before landing on the line the song is built to reach: "Vale madres la vida / Antes de ti" (life was worth nothing before you), and the coarseness of vale madres is doing real work there. It is a shrug, not a compliment.
Around that chorus the song keeps asking to be forgotten and then refusing to let go of anything. It opens with "No te acuerdes" (don't remember) repeated three times, wants the other person to forget "Hubo un tiempo / Que vivías / Sin mi amor" (there was a time when you lived without my love), and then reverses itself: "Y si te acuerdas / Que sea solo / De la gotera / De tu habitación" (and if you do remember, let it only be the leak in your bedroom). The permitted memories turn out to be the intimate ones — a drunken night in Mexico City, a guard let down, and "las dos o diez / Mentiras que quizá llegué a decir / Para subir mi apuesta" (the two or ten lies I might have told to raise my bet). Pink hair, a boyfriend in New York, someone who hardly ever stumbled: the two of them trade the versions of themselves that came before, and neither actually wants them erased.