Doing a wide range of tasks I'm not familiar with, and learning them well and quickly, has done wonders for my ability to just say, 'Fuck it, I'm me and I can do whatever I'm paid to. I've done stuff I didn't know how to do before.'

It also helps to know what the complexity of the task is have little self-affirming narratives - if you know that people who you don't consider smarter than yourself have done something, and have some idea about stacked complexity, then it becomes a lot easier to say something like "This really isn't that complicated, I just don't know how to do it yet, but that guy does it and he's an idiot - and he probably didn't spend years really learning it."

If you can draw parallels with what you already know, that can help too.