I was the same all through school. Great on all of the research papers, but terrible on any that needed creativity. About a year ago I posted a story here, which got about six chapters in before I ran out of ideas and sort of just stopped it. It was a decent experience, and I would recommend giving it a try, but I know as well as anyone that it isn’t for everyone.
I hated English at school (that was a LONG time ago!!), and was never any good at creative writing, I was a "techie". Over the years, I've needed to write things to explaining technical things to not so technical people (I think that's called 'product documentation'! It was never really as part of my job, but in my volunteering work).
So I decided to try writing a braces story on here.
And yes, as you say, as an inexperienced writer, I would run out of ideas after a bit, plus have problems actually ending a story. Whilst a few of my stories do have a sort of ending, many have ended abruptly for one of many reasons.
If I were professionally writing a book I'd worry, but here it doesn't matter. You may have a story that only fits one or two posts, and that's fine, so do be encouraged and give it a go. The second time, you might be able to make it a little longer.
If you're worried about making the story look ok on the page (ie the formatting), then don't worry, Braceface2015 (Aka "the Archivist") seems to enjoy doing that: just write, and let him make it look pretty!