Why are my nails thin, flimsy, brittle, peeling, grow downwards toward finger, "ridged" with vertical lines, break easily? toenails aren't like this.

Many possibilities : So there are many possible causes of nail disease. Common causes are vitamin deficiencies, skin diseases such as psoriasis, infections, etc. The curvature of the nail toward the finger might be something known as clubbing. This can occur in lung or heart problems and is common among smokers. Might be best to consult with a dermatologist.