My biggest regret (not really disappointment) with anything I've ever written & gone on to post somewhere, is not waiting until I'd finished the whole thing before posting. I'm dealing with that issue right now regarding several fanfictions.
I want to get the story out into the world & I know that I need to write it all out first. My writing style demands it. I am not the person who can plot-point each chapter, from start to finish, and work with an outline. I can't & I don't. I do not write based on any kind of preconceived ideas. I have no idea where a story is going until it gets there. The down side is the ease of 'blockage'; if a character is stuck so am I & that often leads to days, if not weeks or months, of a story collecting dust until it can move forward again.
I love the way I write, it's exciting to see lives & worlds unfold before me, so I have no regrets about that, but my issue is not allowing the process to play out in full. Fortunately, I am better about it than I used to be.
At this point, I've got to get myself together & tie up all the loose ends I've left over the years so I can move ahead without feeling guilty for leaving readers with cliffhangers for months at a time. I've got three stories to finish & I want to do them justice because I enjoy them, the readers enjoy them, but I'd hate to rush any of them just so they're 'complete'.
I guess I'm disappointed that I left them hanging because I wasn't able to be patient with the story lines, but I do plan to fix that.