why does parts of my hair curl and the other parts remain stringy?

Okay so, my hair is definitely a situation. I loved my curly hair when I was much younger but now I hate it. When I went on vacation, I would notice that after washing my hair it was very stringy and thin and nothing like it was before. I did suffer some heat damage when I basically fried my hair for weeks with a flat iron. So I made the decision to chop off my hair, and now it's a little past my shoulder. I thought that would resolve it but just today I washed my hair and some parts are still stringy. Yes I still use heat on my hair, twice a month, and honestly I don't think I'll stop especially now that I have short hair. Please help me and thanks for reading.

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