I'm looking for a manga that has a male lead with a sad past and a quiet, shy and closed off personality, possibly romance, no yaoi, yuri, harem, ecchi, smut or cross dressing, something like Iris Zero, but with more romance.

Mmmm......well, I can't swear all/any of these match up exactly with what you're looking for, but I'll try to throw some (of the better ones that I know of) with at least kinda "messed up" male leads at you 🙂

3 Level Combination, Aku no Hana, Attaque, Come Spring, Hanged Doll, Hwaja, Sakamichi no Apollon, The Friendly Winter and Yuutai Nova.

Huh, they turned out to be more webtoons than manga....must be a Korean thing I guess 😀

All completely scanlated. Good luck finding something 🙂

Bastard (HWANG Youngchan) is a perfect fit I quess. MC had horrible past that made him sort of an outcast at school. He doesn't know much about the real world and has communication troubles. But there's great character development in this + subtle romance going on, so enjoy 🙂

Sekine-kun no Koi - I think he might be older than what you're expecting, but he's just too precious.