How can I cure my 4yr old boxers itching problem ? could it be psychological ?

For the past year our boxer has been itching, licking and biting her paws and face uncontollably, she licks so much that she develops lesions and bleeds. she eats dry food. (however we also tried canned food for a while) On our vets suggestion we have tried changing her food, from Eukanuba to Hills and several other medicated ones for sensitive skin or for skin conditions, we have taken skin scrapings and blood samples which all come out negative. We changed her vet 4 times but they all seem to have run out of options to try. we have also tried changing detergents used in the washing machine and in the water when washing the floor. Other then her paws and face where she itches she has a very shiny healthy coat and has no other medical problems. we recently asked our vet whether it could be psychological or a habit and (i think out of desperation and not knowing what else to tell us) said that it could be. if so how would we deal with it ? she is given lots of attention and love