Anaemia, or at least not enough iron-rich food in your diet soop?
Long nails are inevitably weaker than shorter ones so keep them neatly trimmed. Biotin (vitamin B7) helps to strengthen nails. You can take a supplement or find it naturally in brewer's yeast, egg yolk, offal and soy products. Coat brittle nails with warm olive oil plus a couple of drops of lemon juice at bedtime and let soak overnight. Rub soft nails with a mixture of cider vinegar and a few drops of lemon juice once a day. Don't use nail polish remover that contains acetone.