I don't know if i'm scientifically correct in my thoughts but here goes.. I don't think that any of these "strengthening" nail varnishes work as you are painting onto the dead nail and in my own experience, nail varnishes usually dry out your nails even more, causing more splitting/breaking.
I would say that you need to increase the calcium in your diet (i'm not a doctor or dietitian btw, just a hunch) or use a cuticle oil. I'm sure my mum uses a Vitamin E Cuticle Oil from Sally Hansen and her nails have gotten a lot better. I'm not sure where this is available to buy though. I'm sure that there are other good ones on the market.
Hope this helped a bit :) x