I have written some short stories (in fact, they are flash fiction). When I'm writing short stories, the ideas and the structure come to my mind by themselves.
Something different happens when I try to write long stories; It starts with something simple, but then I add more and more stuff until it gets too complicated. Another thing that happens is that I lose interested in the story after some weeks. I just feel that it is not a story that I want to write. Other times, I just tell myself: I have nothing else to say in this story, it should end here.
Does this happen to a lot of beginners? How to overcome this problem?
(Sometimes I think that I should just accept my fate as writer that only writes short stories. Or just write novels that are composed of interconnected short stories, like David Mitchell's Ghostwritten.)