It's a romance shoujo manga. There's this rich guy who has a high school bodyguard whose a girl and he loves her. But she can only be his bodyguard until he is 18. Later he's kidnapped and the girl goes and rescues him but there's a bomb planted in the building. The girl confesses her love before sending the rich guy down the elevator and the bomb goes off.

it's not grand sun, kimi wa boku no toriko nare, or shoujo guard.

this is someone else's description but it's bugging me too! i so remember reading this. if anyone has any idea, i would be so grateful. i've been searching for hours and i can't figure it out.

someone on another forum found the answer for me.

it's Invincible Love Mission, a oneshot in Anata e no Crescendo.
http://www.mangaupdates.com/series.html?id=14166

you can lock this thread now.