Hi. I definitely understand that premature white hair or "canitis" as they are called in dermatological terms can be a cause of significant mental distress. This can occur due to genetic and hereditary factors, thyroid disorders, environmental factors and stress. Have you seen a dermatologist.? Am sure he'll be able to help you with the tests to be done - if necessary and also necessary medicines.
If there's a significant genetic component, then it may be tough to reverse greying. Hope this helps- let me know if you need any further assistance in this regard. Take care