"I’m not the brightest light in the bulb."
It shows. When you mangle a simple yet cliché adage describing yourself as a dim bulb, you prove the saying as true. Are there any other aphorisms we can fumble?
Not the sharpest color in the crayon.
Not the only tool in the shed.
Why not just say "I'm not that bright." At least then we could give you the benefit of doubt and assume you're being humble.