Should i tell my manager i might have adult ADD?

So I work at an oil change place as a cashier and lube tech. I have a lot of trouble concentrating because of my severe brain fog i get through out the day. Ive been having this problem since high school i believe. But anyway, while he is training me on new things he is getting frustrated with me being so forgetful and dumb. Things that he explains to me i just dont seem to understand. And i make a lot of really really dumb errors that make me seem irresponsible and clumsy. I'm sure he thinks im an idiot. But the thing is i have a real attention problem and really cloudy brain. I'm thinking if i should just tell him about it so he will understand why i am like this. But then again I'm afraid of what he might do too.