Is he held accountable who is reminded by his friends of his old sins from which he has repented, and he fears their reminding him of them when he enjoins good and forbids evil?

Congratulations on your repentance and on being granted success in memorizing the Qur’an. Do not pay attention to people's taunts, for enduring their taunts may raise your ranks and magnify your reward. If your advice to them does not succeed and they continue to taunt, then there is no sin upon you in ceasing to advise them. It is recommended to change your methods of advice, such as speaking with each person individually and offering gifts. And remember that believers are brothers.