Usually, we express "my worst terrible dream" as "my worst nightmare."
So, if your worst nightmare is going to the toilet (and, at least in the U.S., we'd put the article "the" in there), you would say:
My worst nightmare is going to the toilet.
But your sentence doesn't talk about your worst nightmare, it talks about the nightmare you are most afraid of having. (The two aren't necessarily the same thing. For example, my worst nightmare could be the one where my wife runs off with my best friend, but the nightmare I may be most afraid of having has me going to the bathroom – because that's the one that makes me wake up needing to go to the bathroom!)
Anyhow, you could say that as:
The nightmare I fear most is the one where I have to go to the bathroom.