As I am new on this site I don't fully know what is expected from a reply and I certainly don't want to come across like I am saying this is what you should do etc, this is just from my experience and I am not sure how much it will work for you:

The quote 'why open an umbrella before it starts to rain' comes to mind. I know it is very easy to worry about everything and then the worries just seem so big you wonder how on earth you can cope with them and I think it is very scary! If you can deal with each worry, each concern at a time and maybe write a solution to it. So you say you are afraid you won't be able to become a teacher, can you write down why that is? Then objectivly write solutions to each thing? I am not sure if that is applicable for you but I find it helps a lot.
Also, you mustn't think that you are not interesting or smart enough - I know it is so easy to write it down but so hard to believe it - but try to have faith in yourself, find examples of good work and repeat all your achievements to yourself, maybe that will help?