Why do people overestimate me?!

This is getting really frustrating. Stayed at my grandparents' till today and yesterday, my grandma was asking me what I want to do next, work or study further. When I said that I wanted to work and see where I'll go from there, she was a little insistent on studying a little more. As much I appreciate that, I'm not that smart. Forget smart, I'm not even close to normal intelligence. It takes me ten times the effort just to be on par with something that that's easy for other people. I just can't process and think as fast as normal people.I try to explain that to her(not the last two sentences) and she says "Just because things went bad this time, doesn't mean that they go bad everytime, does it?".

But she doesn't get the point. I'm not smart. And with my luck, there's no doubt that things will be atleast as bad as this time. There's no point trying. I'll just be a complete failure in another country or city. Why doesn't she understand that? It's just not worth it. I'M just not worth it. I just hope they realize that in time; before it's too late. I know they say that everyone has a strength, but I just don't.