I am working on a mathematical topic that is basically my own concoction. I have days where even the simplest thing takes ages to do and I realize that's because, deep down, I am terrified that things might not work out. This is in different way: I am scared my topic might not be novel enough, appreciated etc. ; I am also scared that checking parts of proofs will reveal catastrophic mistakes. 

I rationally realize that this is part of the game and I do my best to take mistakes I find as good news that my work is improving, but at times I am just paralyzed and unable to do things.

I am also bothered by the presence of colleagues in my office, afraid they might judge my work (and at the same time going to a hidden place in the library helps a little but is also very annoying and uncomfortable). 

I am sure this must be, to a different degree, experienced by many PhD students. 

What are some strategies to fight fear as you work? 

Note that I realize there really is a deeper confidence issue and I am already seeking help for that. I'd be happy to hear considerations on the possibility of therapy or counseling and how it worked for you as well as day to day tips and strategies.