I chose letters instead of answers for the security questions and now they don't work, how can I fix this?

See if you can reset them : If you forgot the answers to your Apple ID security questions

If you can’t reset them yourself contact Support in the country where you are (and which is therefore the country on your account) and see if they can reset them for you : Contact Apple for help with Apple ID account security

If your country isn't on that page try this form to contact Support : https://www.apple.com/emea/support/itunes/contact.html