Sometimes I have some good ideas when I start writing a new hub, but then I get this lack of ideas and that's when I get very frustrated. But, some other times my brain creates really great ideas while developing my hub.
The ending part of writing a hub is my favorite part. Every published hub is like a mini accomplishment. Like you, I tend to start a lot of hubs with great ideas but they kind of fade when I actually start typing it up. It sucks when that happens but you learn from those down times as much as the times that ideas actually work out.
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