Condenaste comes from the verb condenar, meaning 'to condemn'. In this song, it's used in the past tense, 'you condemned yourself'.
It's a strong and dramatic word that highlights the consequences of the ex-lover's actions. The singer uses it to emphasize that by leaving him for someone else, she sealed her own fate of regret and loneliness, implying she brought this negative outcome upon herself.
This is a breakup song that refuses to leave the room. The narrator has been cheated on, says so flatly in "Te fuiste con alguien más, sin importarte" (You went with someone else, not caring), and then spends the rest of the track explaining exactly where he intends to be from now on: "Ahí voy a estar cuando te duermas en sus brazos" (There I will be when you fall asleep in his arms). It is not a threat and it is not a plea. It is a promise to be the thought she cannot put down.
The best bit is the one about the moon. "Ahí voy a estar cuando la Luna no te brille / Porque ella sabe lo profundo que me heriste" (There I will be when the Moon doesn’t shine for you / Because she knows how deeply you hurt me) hands the moon a grudge on his behalf, which is a bit much and works anyway. Notice how often "ahí voy a estar" comes back, always in the future tense, always about a moment that has not happened yet. By the last verse he has stopped arguing altogether and just states it: "Sola tú te condenaste" (Alone, you condemned yourself). He gets the last word by never asking for one.