Downward Growing Fingernails

For a couple of years now 2 of my fingernails on both hands have been growing downwards at the end of the nail and have little lines going vertically across all nails.
I also have very strong nails apart from the 2 nails with the problem; they seem to be very weak and bendy at the tips.
I have read a lot of things about it like, it could be heart or respiratory problems or a deficiency in vitamin B12.
If anyone has information or advice on what it could be that would be great.
Also if it is deficient in vitamin B12 what sort of food could i eat?

:)
 
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