How does a young man get rid of feelings of admiration for a girl after repentance, and what are the legitimate ways to overcome that negative effect on his heart and conduct?

You can rid yourself of infatuation by repenting to Allah, striving in supplication, choosing righteous companions, continuously remembering Allah, and recalling death and what comes after it. Allah guides those who strive against themselves: "And those who strive for Us – We will surely guide them to Our ways. And indeed, Allah is with the doers of good" (Al-Ankabut: 69).