Another thing that can happen is realizing you simply haven't got the writing chops to be able to write your story well. This isn't an admission that you will never be able to write it, only that you can't currently write it. I had to set my "magnum opus" aside a few years ago because I simply couldn't write it- it was too broad in scope for my knowledge or skill. It's going to take a lot more knowledge of history and culture to be able to write it well, as well as a better grasp of language and an ability to write a lot of interweaving events.
But I didn't just drop it, I decided to get out and write other things, in order to get what I needed. And now I'm so busy on the new stuff, I haven't even taken notes on the other story in about a year (not to mention that the high fantasy field is already saturated and it really doesn't seem as original now as it did then
). I started with a story I knew I had a much better chance at getting right.
But you can't learn this stuff without banging your head against a lot of walls. Repeatedly.