What should I do if I think I am a terrible person?
Why do you think you're terrible?
I've been bad to people, I've been bitchy and self-centered.
I understand how you feel, but being mean to yourself and calling yourself a terrible person isn't going to help anything. You should be kinder to yourself, and learn from the mistakes you've made in the past.
What if being bad makes me feel good.
You should love and respect yourself, instead of putting yourself down. When you can do this, you will be able to be good to other people too.