Find a mentor who can give you candid feedback, even if it's feedback that you do not especially want to hear. Send this person your long form manuscripts, even if you have to bribe them to read them, and ask them to pick apart your novel-in-progress or whatever it is you're working on. Such line-by-line, sentence-by-sentence evaluation is the best way to learn and, after enough of their candid, well-considered editorial feedback, you'll learn to edit yourself, even if your mentor's feedback is painful to hear in the short run. But you have to find a mentor who cares about story and about story telling, and who can give you the right kind of feedback, without crushing your soul. It's a delicate balance.