When we don't really really really believe God's promises we are calling him a liar. When we give excuses, say "it must not have been God's will" we are letting ourselves off the hook to not hold on and belief even when everything is saying what we are believing for is ridiculous and will never happen.
God says what He means and He means what He says.
Some people think there could be errors in the Bible because it was written by people. I don't agree with this but even if you do, then just use the red ones (the ones Jesus is quoted in). So, let's take a promise of His (and that means it is Father God's too because Jesus said what He spoke came directly from the Father):
"'And all things you ask in prayer, believing, you shall receive.'" Matthew 21:22
God said that. Do you believe it?