I had the same problem with both of my big toe nails. Both of them became thick, and a very pronounced curve, resulting in ingrowing problems, and making it difficult to cut.
I now use an oil treatment made by Gehwol (available on Amazon), which was recommended to me, and it's also suitable for diabetics (who have to be very careful with their feet). They do a range of products including fungal treatment, ingrown nail treatment, toenail fluid, foot cream, etc., and they all care for both skin and nails, and help make the nail more flexible.
I filed the surface of the nail initially, then applied it twice a day. There was a noticeable difference after just a few days, with my nails becoming much softer/flexible, and straightening out again. I've used it for a couple of years now, but if I stop using it (I forget sometimes if I go on holiday!), the nails begin to curve again. It's become part of my daily foot care routine after I take a shower. The products are very good, so I'd recommend giving them a try.