How can a Muslim rid himself of recurring sins after every prayer, and how can he continue to receive the divine grace that sometimes transforms his evil intentions into good deeds?

Conversations between young men and women online are forbidden (haram), and they are among Satan's means to mislead people and ensnare them in forbidden desires. Good intentions do not justify prohibited means. You must fear Allah and repent from these conversations. It is incumbent upon the one who bears the Qur'an to distance himself from doubtful situations and sins, out of reverence for the Qur'an, and to utilize his time in what benefits him.