Alejaste comes from the verb "alejarse," meaning "to distance oneself" or "to move away." It's a poignant conjugation because it directly addresses someone, making clear it was their choice to leave.
In the song, the line "Alejaste tal como me diste amor me lo" ("You walked away just as you gave me love") captures the pain of someone who gave love freely only to watch the other person pull away just as quickly.
This song is a powerful journey through the pain of a breakup. The singer looks back on the sweet beginnings of a relationship, remembering the "primer beso" (first kiss) and the "primer mes" (first month). But now, she's faced with a heartbreaking contrast: receiving the "último beso" (last kiss) for the very first time. It's a confusing and lonely experience, as she wonders if her ex made her fall in love only to leave her all alone.
The lyrics reveal a deep internal struggle. On the outside, she tries to pretend she's moved on ("por fuera me engaño" - on the outside, I fool myself), but on the inside, she desperately misses him ("por dentro Te extraño" - on the inside, I miss you). He promised forever but then cut things off, leaving her with a love that was given and then suddenly taken away. Even after a year, the pain is still fresh, and she doesn't know how to erase the memory of "you and I."