The last, physical, manga I probably read was Paradise Kiss. That had to be six years ago or so? Now I'm back at it and I'm on a romance/josei reading marathon. However, I feel like I've hit a wall and I can't find something I'm super interested in.

Manga I've enjoyed:
Mitsu x Mistu Drop
Private Prince
Midnight Secretary
Velvet Kiss
Kyou Koi wo Hajimemasu
Desire Climax
Peach Girl
Kimi wa Petto
Fuushigi Yuugi
Say I Love You

Ultimately, I like smut (even leaning toward ecchi) with a romance storyline. I don't mind manga with less sexual content, I just like "realistic" relationships and art styles where the characters don't look 12 years old.

Any recommendations (please, no one-shots or yaoi/yuri)?

Hapi Mari?
Udon no Onna

Nana, I suppose. Only if you're the type that likes lots of relationship drama. Same person who made Paradise Kiss.

Try Erotic Fairy Tales. Smut to the max.

It's seinen but have you checked Hetakoi ? It should fit what you want I think.

Tomodachi Ijou by Okazaki Mari, and just anything by the woman in general, like what we're working on, which is -&-. Take a look: http://stilettoheelsteam.com/comics/reader/read/_and/en/1/1/page/9

I also think you will enjoy Ane no Kekkon as well.

SkyDewa mentioned Udon no Onna, and I definitely agree with that recommendation.
😃

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Anything by YOSHIHARA Yuki, the Queen of funny smut -- Haa Haa, Aisuru HIto, and the only one published in English so far, Chiyou yo Hana yo (Butterflies, Flowers)
Oishii Kankei - not smutty, but a really good realistic story
7 Seeds - more adventure than romance, but a great read, by mangaka of Basara
Soryanaize Darling
Yuru Koi - only one volume, but I prefer it to her 2v Shuukatsu!! - Kimi ni Naitei
Candy Life - only one volume, by mangaka of Kimi wa Pet
Kuragehime - I haven't read it yet; everyone's talking about this since it got an anime, but I don't think it has any smut and I'm not sure how much romance
Second Hapi Mari, also in English (Happy Marriage) by mangaka of Private Prince. She also writes terrific smutty oneshots if you get in the mood for those

Mature shoujo:
Kiss
The One
Teacher's Pet maybe, lots of smut
Provence, but it suffers from having two separate stories
Honey na Koto
Lovenista maybe

SHINJO Mayu - love her or hate her, if you haven't read any of her works, you should try it for yourself. Kaikan Phrase is probably her best work, Akuma na Eros is an absolute howl.

Dare nimo ienai and Mars no kiss as the "mature" josei I've read and can call to mind.