Why does itunes keep saying my credit card was declined when it wasn't declined at all

Either it's not passing iTunes validation or your card issuer is declining it for use with iTunes.

For a card to have a chance of being accepted it needs to be registered to the same name and address (including format and spacing etc) that you have on your iTunes account, and have been issued by a bank in the country where you are (and therefore the country which is on your iTunes account).