How to switch iTunes account from credit card to iTunes gift card?

I already entered the code on my card and it said that I have successfully activated the card, but whenever I try to buy an app it sends me ti an edit payment page. When I say that I'm using a gift card to purchase the item it wants me to activate the card again, but since I already did it won't let me... show more I already entered the code on my card and it said that I have successfully activated the card, but whenever I try to buy an app it sends me ti an edit payment page. When I say that I'm using a gift card to purchase the item it wants me to activate the card again, but since I already did it won't let me activate it. So now i cannot nuy my apps with my gift card. On the edit billing page it also says that my credit card has expired so I need to select a different form of payment. I click "none" and then I click on the iTunes gift card, but it says that whole activation thing and how I cannot redeem the card because I did already. But by my user name on the top right corner, I can see my $25 dollar credit. What do i do???????????!!!!!!!
Update: um, when I go to the edit payment page it says that i have no credit card number for it expired, therefore i have no number to delete. To be honest, i think i totally screwed up and just wasted $25 on a stupid iTunes card. this sucks =(
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