The first question to be answered is, why can’t you take a deep breath? Is it weakness of the respiratory muscles?; the, perhaps unconscious, fear of pain if you do?; is there a feeling of obstruction to inhaling that cuts-off your breath, before it is complete?; or something else that you feel when you get to the point where inspiratory air flow stops?; could you have weakness or paralysis of one of two of your hemi-diaphragms?; has a chest x-ray been done to rule out something that might be blocking one of your bronchial tubes?; and have pulmonary function tests (PFTs) been performed?
You should discuss these considerations with your doctor.
Good luck.