How do I know that this website isn't a hallucination/delusion? How do I know that my therapist is real? How do I know that any of this world is actually real?

How do I know that I'm real, for that matter?

I don't know the answers to any of those questions. I can't prove that I'm real. Nobody can prove that anything is "real". So why have so many people told me that my perceptions are bull**** and couldn't possibly be real, and yet they can't prove one darned thing about their so-called "reality" actually being real?