I'm doing GCSE german currently and I'm wondering if there was any easy to way to learn cases quickly when speaking in German because i am able to use them in my writing as i have chance to think about which one to use however in my speaking i am forced to use them spontanteously and im really struggling :/, any tips would be much appreciated! :))
Find out when exactly you use each case and in which situations you need to use them. Then just keep practising and practising and it will eventually sink in and come natural to you. It doesn't matter if, at first, it takes you a while to think of which case to use or if you make mistakes. With practice comes perfection :D
I can only second what's already been said, I tend to have the same problem as you, sometimes when I speak the grammar goes out the window, it's just a matter of practice.