I do like trees. you know if you figure the percentage of the tree that died, which is about 25%, you have lost quite a bit. Willows are always in need desperately for water, that is why there found along creeks and rivers. Willow limbs are not real strong and heavy winds will crack them easily.
Something is definately wrong with your tree, being so young and lost such a large amount. I would seek an arborists or just dig it up and re plant another healthy one. You have not lost much growing time being only 9 feet tall.