There is a vitamin your body is needing that it's not getting....
I used to have this problem, too. I tried using Hard as Nails, a clear polish. It might be a vitamin deficiency; you could try a health food store and ask them what is good for strengthening your nails. I can't remember what else I used/too, but some particular vitamin. Some foods will help, too, but the health food folks should be able to help you
Nail quality is related to your health. Have you had labs? Because I have found dermatologists just treat symptoms, and are not interested in guiding people as to the cause of their skin and nail issues, I would start with your primary care provider. You need to be checked for vitamin deficiencies and causes for malabsorption, anemia, thyroid disease, etc.