How do you convince someone the world is round when they KNOW it's flat?

I have thought of bringing this up many times and I never do. I believe, no I know, I'm a worthless piece of trash. People on here and other places say things like No you're not. Youre a kindhear person. They are either saying that to be nice or don't know the real me.

Just keep telling yourself you're not worthless. Think about the last time someone said something so outrageous and stupid it made you laugh. That's how I feel about that. I'm not worthless. Haha don't make me laugh. You know you are a low life POS.

If you just goto a therapist they will help you with that. I'm too paranoid (for good reason) to trust someone and tell them about my problems. I can point out time and time again when at best nothing came of it.

So how does one convince me of something I know beyond any doubt that it's not true? Like I said if I believed the world is flat, how do you convince me its not.