Konrad, you may have depression, which makes you think so negatively about yourself.  Do you have a school counselor you could talk to?  Maybe he/she could suggest a doctor or a therapist you could see and maybe you even need medication.  Adolescence is a tough time of life, I know, and it's worse if you're depressed on top of it.  Can you talk to your parents at all?

I hope you will reach out for help - writing here was a good start and there are lots of people here who would support you.  But this site is no substitute for face-to-face help.  I hope you'll think about it, anyway.  And please write again and let us know how you're doing.