I Love YOO

Hmm, okay...here are my recommendations. (You may have already read some of them!)

Colette wa Shinu Koto ni Shita
^ I thought of this one because it's a quiet, slow-paced romance where the relationship really builds, like in Horimiya. It's set in a fantasy world, not the real world, but it's very focused on personal growth as well as the relationship.

Wotaku ni Koi wa Muzukashii
^ This one's a real-world story, I'm just recommending this one to basically anyone looking for shoujo/josei romance stuff because I think it's amazing, LOL. It's so cool to have a story about actual working adults that is really fun and nerdy. This just got adapted into an anime, too, which has some really good execution even if the animation/art budget wasn't super high..

Dame na Watashi ni Koishite Kudasai
^ Also real world, and also slow but there's a good sense of humor between the MCs. :) Also about grown-ups, but the MC is a bit more childlike and goofy, which may not be to your taste?

Koi Dano Ai Dano
^ OTOH, if you like protagonists who are really independent-minded and strong on their own, Koi Dano Ai Dano is the way to go! It's actually a continuation of a manga where the MC and LI start out in middle school, but it's less about their romance and more about the MC growing as a character and making friends and stuff. This high school continuation keeps on from there, but is a little bit more about the MC/LI relationship and other romances. If you really want to start from the beginning, the original is: Warau Kanokosama

And finally...

Tonari no Kaibutsukun
^ Another good no-nonsense, strong female MC! Also high school set. In fact, if you liked Horimiya I have a feeling you've already run into this one, it got an anime adaptation so it's got a higher level of awareness. But if you haven't read it, you should. :D