I occasionally go through periods when I lose confidence, feel flat like you said, and have trouble breaking out of it. For some people, I think it's all just part of the emotional cycle of highs and lows.
I actually wrote about it a ways back:
Overcoming a Loss of Motivation.
I'm not sure if there is any way to snap right out of the Yuck. The best you can do is understand that it will eventually pass and try to keep going without getting too down on yourself.
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