

i have something that is similar
http://www.mangago.zone/read-manga/the_strongest_girl/

Happy-Go-lucky female lead with a dark past?
Like in Deadman wonderland or Code Breaker.

Fruits Basket. A sad past, not neccessarily dark. A classic.
http://www.mangago.zone/read-manga/fruits_basket/
Shounen/Seinen manga with best love story?
http://www.mangago.zone/read-manga/psyren/
http://www.mangago.zone/read-manga/rave_master/
Sadly I've read both of them already
Seinen - Sekine-kun's love. HANDS DOWN. I've never cheered up for a manga character's love life so badly than him.
shounen - nisekoi (this is harem tho but i am into harem) i actually really like this, the concept of "sawaze in love" key-thing is romantic.