I'm looking for a manga in which in the beginning of the manga the main character confesses their love to someone and gets horribly rejected. Not a simple "Thank you for your feelings, but I don't think of you that way" but more of a sadistic "HAAHAH you think you're good enough for me? look in the mirror you ugly talking dog!"

Basically the rejection needs to be soo horrible and scarring that the character is psychologically effected by it.

this kind of manga will probably be a shoujo, but I don't want the cliche shoujo where the girl gets rejected, "doesn't give up" and changes herself to get the guy.

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Feed your head.

Just go grab Girls Saurus. You'll see a really epic rejection in there.

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I was going to suggest Skip Beat,
until I saw that last paragraph of yours.
But I did anyway, so you decide.

I've already read it and it doesn't fit. The guy who does the rejecting is psychologically traumatized not the one who is rejected. Although she does go through a major change because of the rejection.

Although with skip beat its not a rejected confession, it still kinda works because its "Rejected love leading to psychological changes".

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Feed your head.

Perfect Girl Evolution.
But due its popularity, I assume you've read it?

Lovely Complex (in a way...? xD)

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(c) credits go to shizuu for the avatar!

Sarasah maybe? It's a good manhwa, but there are just 3 chapters in English.
The rejection ends very badly... the girl doesn't really give up but she doesn't change herself because of this guy, it's more complicated