Novel =/= Short story

So, I've been writing since I was very young.  The problem has never been, "What to write about next" - but more along the lines of keeping up with the last thing I wrote about. 

As in, all I ever seem to get are half stories (the longest being about 20 pages). Any tips for actually writing something substantially longer,  instead of ending up with a short story?

You have been writing short stories for quite some time? Want to tell a longer story? Use the short story sequence format, where your short finished pieces follow the plot, but not the form (as in, the story is continuous, but composition is every time a new thing with each piece having it's own beginning and end, unlike that of the chapters in the novel, which aren't compositions on themselves, but parts of a composition).

Now, if you really want to write something "long", you should consider that it takes accordingly more material to keep things substancial. Just writing longer about stuff isn't going to do, or you'll end with just another shitty novel, of which we already have too many.

The difference betwen a novel and a short story is that the short story is more of a picture or a short cause-result sequence. Now, the novel consists of several different happenings which, _related to one another_, constitute an aesthetically pleasing combination. The is: the novel should ALWAYS be more than sum of it's parts. You may be writing about however small things, but together they should form a really grand (aesthetically, emotionally or other) picture in your readers imagination. Sure, some writers just write overly long stuff simply because novel is an established format: that be mediocre and bad writers. Good ones, on the other hand, write the shortest they could, and if they write a novel, then it must be the shortest they could to express a poetic image convincingly and without preaching like a faggot.

So, simply put:
- got a long story? Use a sequence of finished short stories to tell it.
- got a complex idea? Are you sure you aren't just going to waste your time on somehting you aren't all that interested in? For this your "complex idea" being the most interesting stuff in the world for _you_ is a must and an absolute prerequisite. Yes? Fine, then: start composing, with this in mind:
1. The purpose of your whole book is expressing this one artistic idea; everything else is disposable.
2. Composition is your instrument for driving _it_ out just as you have it in your mind. Every scene should be moving you _there_. Every major character should be there to imprint upon the reader the bigger picture; don't forget the next point, though:
3. The only elements allowed to be slightly outside your main goal are things that make your story more convincing. Character development, for example: so that you wouldn't have cardboard cutouts that could harm your big picture with their ugly presence, you know. Some dramatic or awe-inspiring happenings, too: so that you could pre-inspire you reader and feed him some awesome before you hook him, perhaps.