Is the supplicant's saying in his supplication, "O Lord, I humble myself to You," unlawful or inappropriate?

If the supplication is "O Lord, I humble myself before You," then there is no legal impediment to it, because its meaning is submission and fear of Allah the Almighty, and these are required matters in supplication and worship. Allah has commanded us to supplicate with humility and in secret. Supplication is the essence of worship, and Allah has promised to answer the Muslim's supplication as long as it does not involve sin or severance of kinship ties. It is recommended for a Muslim to supplicate with the reported supplications, while it is permissible to supplicate with whatever is easy.