I cannot simply decide that I no longer believe something exists
If you could make yourself disbelieve NSA, would you?
"Can't I just let myself forget what you've told me? Can't I just let myself forget what you've made me do. You think I just want another puzzle to solve? Another John G. to look for? You're John G. So you can be my John G... Will I lie to myself to be happy? In your case Teddy... yes I will" (from Memento)
Yeah, for most of us. But that's the thing about the guy in Memento - he can chose what to think and "remember" in the future since he trusts his own handwriting and pretty much forgets everything else.
"However I would be happier if I didn't know it existed"
If you could chose to forget, you wouldn't know it existed, and thus be happier. Souds like the logical thing to do :)
If you could make yourself disbelieve NSA, would you?
"Can't I just let myself forget what you've told me? Can't I just let myself forget what you've made me do. You think I just want another puzzle to solve? Another John G. to look for? You're John G. So you can be my John G... Will I lie to myself to be happy? In your case Teddy... yes I will" (from Memento)