It definitely sounds like someone or something shamed you...it's nothing to do with being smart, I used to get like that (still can sometimes) and at the time I was studying some really hard stuff and getting good marks for it. You poor thing, sometimes we feel like 'disappearing' because actually we want the problem or pain to disappear...and we've identified ourselves with it so we think we're one and the same as the problem - when we are not!
Are there any particular subjects that interest you, e.g. films, travel, nature, history etc that you could learn more about and deepen your knowledge? I did this and found it helped my confidence, as even if I didn't quite know what everyone in a group was talking about I knew about the stuff that interested me. If you weren't smart you wouldn't have come here for help
Do you have a counsellor or therapist? It might be useful to find out the source of this sense of shame and embarrassment that you suffer. It's best to do this with the help of a professional. Then you could work at seperating whatever it was from your life now.
And remember, everybody is insecure about something, it's just some people can either manage it or are good at hiding it. You sound highly sensitive, and that's a good thing to be. It means you feel emotion deeply and can be compassionate - exactly what the world needs!