Sometimes terrible thoughts go through my mind.
Do you know why your mind thinks such terrible things?
I just feel that I'm a bad person and I don't know what to do about it.
Having your mind generate bad thoughts can be hard to deal with, but not doing anything about it will not solve the problem. Find someone that can help you deal with your internal difficulties.
I've tried that but it never works for me.
It's good that you've tried talking to someone in the past, but it's a long process and it might not feel immediately helpful. Don't give up and try talking to someone again and they can help you work towards feeling better and understand the root of the problem.