What should I do about this?

I'm about 15. I've been writing since I was probably 11, and since then I've gotten progressively better with detail and all the points of good writing such as similes and metaphors. I love writing, and my teachers always tell me that they love my writing. My only problem is I get good ideas but I can never write at length. I want to write a novel, but I always get writer's block. Does anyone have any tips? Should I possibly just give up?