There’s a few ways to answer this. Firstly, I think Misuzu’s happiness is conditional to letting go of her attachments to life. My theory is that the past girls were all so determined to not burden others, to break free from the curse, desperately struggling for a happy ending while full of regrets over not being able to live a full life, that they never thought to merely focus on the present. All the other girls were focused on throwing off the shackles of the past, and hopeful for the future. Misuzu may have been the first to simply be content with the present, and accept her death. I believe that’s what makes her different, and what broke the curse.
You could also add in the interpretation that the force opposing the curse has grown stronger over generations. The example here would be in Yukito’s bloodline, with all of the descendants hopes and wishes accumulating in his doll over generations, granting him the power to oppose the thousand year old curse.
Both work, maybe it’s a mixture of both.
