I think I'm a horrible person, but no one seems to realize it.
Why do you think you're a horrible person?
I've done some bad things that no one knows about.
Everybody does things that are not good sometimes but it is not good to think that you are horrible. It will just make you feel bad and not want to do anything about it.
I don't think I can change my behavior, I think I'll always be a horrible person.
Everyone can change their behavior, so can you. It's good that you understand you have a problem, now you need to take the steps to be a better person. It will take some time, but I know you can do it.