Flat irons are deadly for curly hair. I dont recomend using them at all, otherwise youll end up like me. I used to straight my hair at max 3 times a day, i also blow dried my hair, dyed it different colours all the time, and i used to get it thinned way more than 2 times per year!! Then around three years ago i moved, so i decided it was time to give my hair what it deserved. So i went natural. And now i have beautiful curls. So i recomend you do too. Although it took me around to years to get my curls fully back, some people get theirs back in just a couple of weeks.
Firstly, stop using the flat iron and dont blow dry your hair.Then get yourself some gel, hair cream, conditioner, or even mouse ( whichever you prefer ).
Cut down on how much shampoo you use, and really hydrate your hair. Use olive oil on your hair around once a week ( this will soften, and give it alot of shine to it).Then you just need to style your hair natural and not use any kind of heat on it.
Keep it up and stay motivated, dont give up on those beautiful curls