I don't want people to think I'm dumb?

I've had below average scores in the past 3yrs of college...since I didn't study at all. I used to avoid conversations about studies and all. But now its high time and I HAVE to study. So it'll definitely take time. I'm not dumb, I'm pretty smart besides the fact I forget things when they're the most required. I'll say that during a witty comeback to a friend but I'll stand blank when my teacher asks me the same thing. So now, since I'm in the process of studying I won't be able to answer all the time, I'll definitely not know things that everyone ele thinks are obvious. So what do I do to not care, instead of not studying that I've been doing for all these years. Or how can I not care if I'm called dumb or what they'll think of me or about comparision with other people and put aside my ego, and admitting that its possible for me to forget even after studying something twice or even more