I think I'm mentally challenged?

I'm 19 and have been a social drinker for years so I think alcohol has affected me in the long term but I think there is something more serious going on with me. I used to be able to sit down and read something, no matter what it was, but now I can't do ANYTHING without being completely distracted and fidgety. In my most recent jobs all of my managers have basically told me I'm slow and forgetful. It seems like I need to be told/shown how to do something literally 8 times after the first time because nothing clicks with me. I'm extremely clumsy and sloppy and disorganized which is something I am frequently b*tched at about my my family, bestfriend, boyfriend, etc. I just don't understand anything. I don't know what's wrong with me?