Please recommend a hair treatment that improves hair quality.

Since I have wavy hair, using a hair iron is... a daily routine for me.

My hair gets damaged easily because I use a hair straightener every day, and I feel like I'm barely managing to hold on.

I hear it makes hair good, but when I use it a few times, it seems like it's back to the same state... It really doesn't make a difference.

Are there any good treatments? I used Hayashi 91 before and that was so good.

I can't order directly from overseas either ㅠㅠ and I can only buy it at duty-free shops;; *sigh*