It's probably a bit of both, physical and mental. I know exactly what you're talking about ''if I start to think on any other body part it will start to itch"...I don't think that is uncommon although I think it has become more of an obsession for you, no?
My advice is learn to ignore it, or at least only itch the genuine itches (if you catch my drift). I understand that will be incredibly frustrating, but if you carry on caving to all the itches you seem to be mentally creating, your situation will never improve. You'll find the longer you go without itching these "phantomn itches" the less you will get them. Well that's my theory anyway