What is the supplication of Sayyid al-Istighfar (the master of seeking forgiveness), and what is the meaning of "And I am upon Your covenant and Your promise as much as I am able"?

The best supplication for forgiveness (Sayyid al-Istighfar) is: "O Allah, You are my Lord, there is no god but You. You created me, and I am Your servant. I am steadfast to Your covenant and Your promise as much as I am able. I acknowledge Your blessings upon me, and I acknowledge my sins, so forgive me, for indeed, none forgives sins except You. I seek refuge in You from the evil of what I have done." The meaning of "I am steadfast to Your covenant and Your promise as much as I am able" is to remain firm in faith, obedience, and certainty in the Day of Judgment to the best of one's ability. Whoever says it in the morning and dies on that day will enter Paradise, and similarly, whoever says it in the evening and dies.