I had very strong nails when I was younger, but now they tend to split vertically and to peel. I'm pretty sure it is not diet in my case. Lately they've been bad again.
I have to add to suggestions to get your thyroid checked. I had to start taking thyroid about five years ago--minimum dose. When my nails got bad I had the levels checked and sure enough, my levels were low and the dose was upped.
It gets tricky once you get past menopause. Most women I know seem to need some supplemental thyroid eventually.
Keep in mind that it takes 3-4 months for a nail to grow out, so the question is, what were you doing, eating etc. last February and March? In my case I've been under a huge amount of stress this entire year and I wonder if that is affecting hormone levels and my current splitting nail problem.
I did read that a biotin supplement might help. I checked the list of foods high in biotin and I get plenty of that, so I have my doubts.