A yaoi with a fairytale plot...like beauty and the beast or the little mermaid, the frog prince...stuff like that! Thank you for your help and recommendations!

i would like to recommend

asterisk

and

jinjuu houretsuden

its not excactly a fairy tale ..

but these are my prince charmings for sure^^

I'm not sure if you would call it a fairytale exactly but I think it's kinda like it.
It's about a guy who meets and helps an old man on the street and that man offers him work (which he needs so he takes it) and it turns out the man has tricked him into becoming the fiance to the crown prince.
I dunno if it suits but either way you should read it cuz its good 😀
Royal Fiancé

Lost Boys by Itsuki Kaname?
Gensou Shounen Koimonogatari by Himawari Souya,
Oujisama ni Narenai by Kamon Saeko, maybe?
Oujisama no Kiss by CJ Michalski,
Usagi no Oujisama by Mizuki Kana,
Chapter 6 Kingyou Hime (Gold Fish Princess) from Inu mo Arukeba Fallin' Love by Takashima Kazusa,
Chapter 4.1 (extra) from Denkou Sekka Boys by Tohjoh Asami.

Sekai wa minikuidesu..shika shi! Yūjō, yūki, tsuyo-sa wa ikinokoranakereba narimasen..Tiruendo!

Mister Mermaid is based on "The Little Mermaid".
"Two Princes" from Kimi no Koe ga Kikitai is based on The Prince and the Pauper.

Heisei Kyouiku Jijou sort of