Is it permissible to supplicate by saying: "O Lord, I am Your humble, lowly servant, so forgive my sins, for You are the Forgiving, the Most Merciful"?

There is no objection to this supplication, as it includes submission, fear, humility, and obedience to God, which are essential qualities for a servant in supplication. This is in accordance with the Almighty's saying: "Call upon your Lord in humility and privately; indeed, He does not like transgressors." It is, however, preferable to use supplications transmitted from the Prophet, peace and blessings be upon him.