Do you know any manga which the girl has painful past, maybe like some guy making her life difficult, she tries running from it, but can't. She restrained her relationship from other people, so nobody gets hurt. Something like this?

bitter virgin?

Yeah, can't think of anything but Bitter Virgin
In a lesser extent, you could consider Kimi ni Todoke i guess, but the manga begins when she actually gets to show less restraint, and opens up to others...

Mars

[img]http://28.media.tumblr.com/tumblr_lq2hczRc5d1r1rvcno1_500.gif[/img]
Monkeys, always at your service.

Quote from NightSwan

I have to thank you again. It's been a while since I've had this much d**k on my computer. :'3

Asa mo Hiru mo Yoru mo
kindda the same with bitter virgin...

oh please do click this!
The sweeter the apple, the higher the branch. The quieter the fart, the nastier the smell.
GUESS WHO??

MARS

cat street
shion no ou
claymorethis one the story doesn't center around the painful past, but all claymores generally have tragic pasts.

Also, it could probably help if you narrow your search by looking at "tragedies". Often I've found that manga included in this genre sometimes have the tragedy part in the past of the characters. I tend to shy away from this genre because I dislike feeling sad, but its not so sad if its already happened to them. In fact it can be uplifting if they overcome the tragedy.

Feed your head.

Kiss in the Blue

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“As he read, I fell in love the way you fall asleep: slowly, and then all at once.”
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Like Skip Beat? Though it's got more comedy than pain in it, but fits the definition...