Believing

"There is nothing either good or bad, but thinking makes it so" - Hamlet..
Yeah, he got that fucking right. Crazy though he might be, he got that one right.

Im stuck with a real problem. Belief.
For example;
Im told to believe that what goes around comes around, but how do I know that? Why should I believe it?
That saying isn't the one I'm having trouble believing, but hey it gets my point across.
How do you just 'believe' that something will happen?
Because telling yourself that it will, doesn't mean you believe it will.

I can say "that is an apple", in reference to a banana. And I can say it over and over again, but I wont BELIEVE it is an apple. I will still feel like its a banana.
Even if i tell myself so much that i really feel like it is an apple, what if sub-consciously I still believe its a banana? Then what? How do I change it to be an apple? I can pretend its an apple as much as I want. I can paint it to look like an apple, or cut it into an apple shape... or both. But that still wouldn't make it an apple to me. I can tell everyone else its an apple, and they will think its an apple... If i tell them that I think its a banana, they say "you need to believe its an apple".
Then what happens if its actually a pear. Am I just fucked then? Or what?

If I cant even believe in simple sayings or thoughts, then I'm never going to make it to Christianity lol.