I'm looking for a manhwa about a fat and ugly duke's daughter that has her engagement broken and then she decides to become pretty, so she starts to eat a bitter leaf and makes a plastic surgery. And after some time she becames the prettiest woman on the empire.

Is it Silver, https://www.mangaupdates.com/series.html?id=18027 ?

Unfortunately, it isn't. I've been looking for this manhwa like crazy, I just can't find it. T_T
But, thanks for the recommendation 😁

perhaps Tenseisaki ga Shoujo Manga no Shirobuta Reijou datta?
https://www.mangaupdates.com/series.html?id=148040

It isn't this one too, but thanks for the recommendation, I'm going to read it 😁

How about this?
https://www.mangaupdates.com/series.html?id=170573

Is it this? https://www.mangaupdates.com/series.html?id=174747
I've only read a couple chapters so not exactly sure how the story unfolds to fit your description

Quote from jinwho

Is it this? https://www.mangaupdates.com/series.html?id=174747
I've only read a couple chapters so not exactly sure how the story unfolds to fit your description

I'm seconding this. This manhwa fits your description.

Been there, read that