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Damon Wayans says he's had enough of working on "Lethal Weapon," so come December ... he's done.

The actor made the stunning announcement to Electronic Urban Report Wednesday, saying ... "I'm a 58-year-old diabetic and I'm working 16 hours a day," so he's over it. He even channels his character, Roger Murtaugh, telling the reporter he's "too old for this."

Wayans says his plan is to finish shooting the initial 13 episodes of season 3 of the FOX series, then officially quit in December. He admits he's not certain the network is on board with his plan.

Damon's dead serious too ... and says he's going to return to the stage after leaving the TV show to "try to find my smile again."

As we reported ... Wayans' time on "Lethal Weapon" hasn't been without controversy. He openly blasted his former costar, Clayne Crawford, in May after Crawford was fired.

Damon blamed Clayne for a stunt that left him bloodied, and claimed he was mean and intimidating on set and enjoyed making females cry. He said staffers referred to Crawford as an "emotional terrorist."