Is it permissible for me to say in my supplication: "O Lord, make me one of the obedient in Your sight"? And is one prayer of repentance sufficient for all sins, or is there a specific prayer for each sin?

Firstly: The meaning of "O Lord, make me one of the obedient in Your sight" is correct if it is intended that one should not fall from the sight of Allah, or that Allah should truly make him among the obedient.

Secondly: Whoever has wronged himself and then repented, Allah will accept his repentance, and the one who repents from a sin is like one who has no sin.

Thirdly: It is recommended for a sinner to pray two rak'ahs with the intention of repentance; these suffice for all previous sins if the repentance is sincere. This prayer is not obligatory but it is recommended, and the one who repents should generally increase in prayer and seeking forgiveness.