How to Overcome Self-Doubt

Self-doubt can be a troubling and persuasive voice that holds you back.
It holds you back from seizing your opportunities.
It makes getting started or finishing things harder than they need to be.
Sure, it can sometimes be useful as it helps you to soberly see your current limitations or simply recognize a half-baked or bad idea. But mostly, it holds you back in life.
So how can you get around that, how can you overcome those times of self-doubt so that you can move forward once again?