Is it permissible to say, "O Lord, enough, avert this affliction from me" due to weariness from supplicating because of the intensity of sorrow and affliction, and is this considered ill-mannered with Allah?

A servant must be patient during tribulation. The forbidden complaint is complaining to the creation, not to Allah; as Ibn al-Qayyim said: "If a servant complains about his Lord to a creature like himself, he has complained about one who shows him mercy to one who does not." Furthermore, complaining was mentioned in the Quran by the prophets Ya'qub, Ayyub, and Zakariyya, peace be upon them.

As for the phrase "khalas" meaning "now" or "urgently," it is permissible. However, it is better to use "I ask You for immediate relief, not delayed." But saying "I'm tired of supplicating" is blameworthy impatience and may be a reason for the supplication not being answered, because Allah loves persistence in supplication. And whoever knocks frequently on the door is likely to have it opened for him.