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.
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
There is a vitamin your body is needing that it's not getting....