I am not proper writer when I can't write short stories?

Of course, this might sound dumb but I just really am not good at writing short stories...I never was, It was horrible or it ended up being novel or whole book....I just can't seem to take all the ideas out on just a few pages (and I envy people who can do it, but honestly I am not interested much in writing short stories) there's the problem though, in many contest and all there's always this demand - write short story on this or that....and thanks to that I never got into any contest because I wasn't able to write anything like that. I even heard somewhere that I can't be good writer because I don't know how to write short stories properly..