I'd like to find a manga with a male lead who had a really horrible event in his past which he haven't really gotten over and ended up with an personality more mature than his the usual for someone his age and somewhat melancolic, and the girl find it hard to get close to him after getting to know of the story.
Would be even better if the traumatic experience was the death of a girlfriend, but i guess this is asking too much already

L-DK , maybe?
http://www.mangaupdates.com/series.html?id=40931

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~And We All Fall Down~

The One for sure!! Both the main guy and his twin brother have bad pasts. I cried!

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"That day, all female officers will be required to wear TINY MINISKIRTS!!!!!"

Bokura Ga Ita Fits to a T
KARE KANO
Also Skip Beat, Fruits basket and Seiyuu Ka-!

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As they played the game...I waited for someone to call "onigiri." But no one called. I was very little then. I had almost forgotten about that. [...] But they knew...there would never be room for an onigiri...in a fruits basket.
-Tohru Honda, Fruits Basket

Hajimari no Niina
That's really cute short shoujo manga. The hero's girlfriend died and resurrected as a baby girl born into a family living next to him - she's named Niina. (She's ten year old when the story starts.)

Torikagosou no Kyou mo Nemutai Juunintachi

I had the same thought (with different spelling). Also, Seikon no Qwaser and Black Cat.

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this looks promising: Shindere Shoujo to Kodoku na Shinigami

Dengeki Daisy should fit
Honey Bitter would be a reverse
Can't See Can't Hear But Love somehow fits

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in need of romance?
uncommon, sad, sweet

orange marmalade maybe?
black bird
Aoharaido
l-dk
bitter virgin
girls of the wild's

try those

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Screams from the haters, got a nice ring to it. I guess every superhero need her theme music...

Kare Kano
Dengeki Daisy
Limited Lovers Both
Skip Beat Both
Mars Both

I realize this is a really old post but try Ousama Game