What should one do who has loved a girl intensely and watched her out of jealousy, despite knowing that this act is forbidden and a sin?

Love between young men and women is not permissible. What is permissible is engagement (Khutbah) followed by the marriage contract (Aqd). Exaggerated emotional attachment is blameworthy. Bearing ill-will (su' az-zann) and seeking out the faults of Muslims are forbidden; this is a sin, not disbelief (kufr). Committing a sin while believing it to be disbelief demonstrates audacity towards Allah and increases the sin. What is obligatory is sincere repentance by abandoning the sin, regretting it, and resolving not to return to it, cutting off the relationship with the girl, increasing good deeds, learning the religion, and being diligent in obedience.