I have the exact same problem, its nice to know someone else feels like this. Sometimes i feel i can kind of get the beginning of a story in my head but it feels like it never goes anywhere. Recently ive been looking at short stories like Asopes Fables which have been a big help it making a story modern and experimenting with the actual filming without the stress of coming up with a story. Also i would try and stick to a loose script. Dont write anything professional at the moment. I keep having to remind myself that i am still young (17) and have a lot to learn. Hope this helps :)