What Causes Weak & Tender Nails?

I am 21, 5ft 7 and am not over weight and I am a vegetarian. I stoped biting my nails about a year ago and wore nail polish for ages to stop me biting them which worked. I noticed my nails were very soft and bendy and were quite see through, so I stoped wearing nail polish incase this was causing it but the condition of my nails still hasn't inproved after a few months. they bend so easily and when they break the top of my nail peels easily.