I lack common sense? Does this mean i'm mentally slow? info inside?

Honestly to this day, I still don't know if I'm mentally challenged or not. I ask my brothers and sisters often if they think i'm mentally challenged etc but they always say no, but sometimes i think they know the truth and are lying to me. Ok, here's some stuff about me.. in school i have trouble paying attention really bad, my mind is always thinking about something else in 2 seconds, i have trouble learning, and soaking information. I think I lack common-sense, I always make easy task very difficult, and make things really complicated. Everyday I go to school I feel like i'm not at the intelligence level of people my age, and that i'm really unnormal but i look normal, i feel like my mind isn't the same like people like me its slow or thinks alot different, i just try to act normal to fit in. Everyday it kills me, cause I don't know if im mentally slow or something or not how could i test myself is there a way? I also am very narcissistic, I think i'm really great, and like a god sometimes sadly i can't help it its a mental disorderrr.... i think so highly of myself and i have no reason really cause im stupid in school, i guess im ok looking, but i have no real talents.