Im trying to find a romance manga where one of the characters has a sad life or childhood.
for instance: parental abuse, terrible illness, school bully, anything sad.

Like erased but with more romance or something, im not sure what to compare it to

This is a fairly common trope, but I'll give you some of my favorite/well known ones that I can think of (mainly shoujo mind you and mostly in abusive parents category).

Bitter Virgin
Mars
Gakuen Alice
Sachi-iro no One Room
Lan chi
Antique Romance
Diamond Dust (KANG Hyung-Gyu)
Eden no Hana
Fruits Basket
Full Moon wo Sagashite
Delivery (not really romantic though)
Bastard (HWANG Youngchan) (my fave)
Kodomo no Omocha (one of my faves, this mangaka has many of this type too)

I'm in idol hell T.T

'm thinking of Inochi and life, the romance is not the main point of these though.

There is also can't see can't hear but love which is a must read.

And fujimi lovers maybe.

I would recommend Toukyou Lastochika.
It nicely combines sadness and romance in a historical setting.

In addition to the many good recommendations above:

Jumyou wo Kaitotte Moratta. Ichinen ni Tsuki, Ichimanen de.
Transparent Cohabitation
Watashitachi no Shiawase na Jikan
Sekine-kun no Koi
Koe no Katachi (may not have enough romance?)
Sengoku Youko (fantasy/historical)
Yukarism (historical)

If you're looking for a superficial kind of sadness skip this one. If you're prepared to ruin your life go ahead and read Zankoku na Kami ga Shihaisuru