Is praying upon the Prophet (peace be upon him) while masturbating considered an insult to the Messenger (peace be upon him)?

You must repent to Allah with sincere repentance for this abhorrent act. Invoking blessings upon the Prophet (peace be upon him) in this situation is not a denigration of him, as it appears you did not intend disrespect. However, it is disliked to mention Allah Almighty while committing a sin. Some jurists have even stated it is forbidden, such as reciting "Subhan Allah" (Glory be to Allah), "Allahu Akbar" (Allah is the Greatest), or invoking blessings upon the Prophet (peace be upon him) in a gathering of immorality or amusement while committing immoral acts.