I used to like a guy.But he was in a relationship.So I prayed for his break up which happened.Now I regret doing so.I was 14 at that time.I did that thing out of foolishness.Although I realized my mistake, I am afraid something like this might happen to me as well which I don't wanna face. How can I repent doing so?
As-salamu Alaykum sister,
The keyword is Taubah. This means repentance.
The religion is ease. Allah SWT says in Surah Al-Zumar, Verse 53:
Say, "O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful."
Therefore, sister, engage in Taubah, always repent. All you need to do is stop the sin, regret the sin and do not return to it and In Sha Allah you will be forgiven. Do lots of good deeds after your Taubah!
You can also engage in istighfar which is a great way to seek the forgiveness of Allah SWT.