I know what you're talking about, I have neurodermatitis (or atopic eczema as it is called nowadays). When it's really bad I scratch the backs of my hands and my forearms untill they look like a bloody mess.
For me it's mostly allergy based, I'm allergic to some kinds of food (cocoa, lemon fruits, strawberries...), to some pollen and some preservatives in cosmetics. Sometimes it's really annoying, especially the food thing. And sometimes it just starts without reason, although stress and anger can make it worse. But I think it's a little better since I started drinking lactose-free milk. I know that lactose intolerance is not an allergy, but I don't question it when it helps.
In winter when it's cold outside and my skin is really dry (drier than the usual dry) I use ointment with evening primrose oil(sorry, don't know if the translation is right, the Latin name is Oenothera biennis), don't know if available in your part of the world. In general I take good care of my skin, but I found out that one of the cheapest skin creams is the best for me. It took me some time to find all these things out, sometimes I was very desperate when nothing seemed to help. And people told me: 'Just don't scratch!'
I hope you find something to help you, I know how annoying that thing is