to those who tell me that it's all in my head;

Yeah, that's quite literally correct. 

It is all in my head.

And that's the problem.

It's in my head, but how in the world can I get it out? 

It's in my head, and trust me,

If I could get rid of it, I would in an instant

Everything is in my head

The fear, the anxiety, the everything

And I want it out