At that rate, sooner or later you're going to be disappointed in everything - either it will turn out not to exist, or even worse, it will turn out to be real.

From a poetic standpoint, I think this sentence is excellent — I love the phrase "even worse, it will turn out to be real"; it makes its point very strongly — but, in case you're using it in your book (which I hope you are), I think that the sentence would flow better and have a higher impact if "turn out to be real" contrasted with a similarly-structured but opposite phrase, instead of "turn out not to exist". Something like "...either it will turn out to be imaginary, or even worse, it will turn out to be real."