I'm looking for a romance about a human and a ghost, preferably with sad themes dealing with how the person died and ended up a ghost, and maybe how they can becomes human again. I want something that deals with the ghost aspect rather seriously, so no shounens or seinens.

instead of a ghost an angel, spirit, shinigami, reincarnation, or anything spiritual and supernatural would work

A few examples I loved:
Return
Full Moon wo Sagashite

hotarubi no mori e was also great but really short

Something touching and relatively long would be great.

Thanks in advance!

There is Tasogare Otome x Amnesia which also have an anime , and Itoshi no Kana which is not bad actually as long as you dont mind the more mature theme 😐

One Winter and its sequel Pinocchio are really good.

There's a Human-Nonhuman Relationship tag you could check out if you haven't already.

As for recs...
Jack, the American Ghost
Aozora Kirai no Usotsuki Semiko (kind of)
Kon no Ki Konoha (oneshot, very like Hotarubi no Mori e)
Inari, Konkon, Koi Iroha. (it's a comedy, but the later chapters does deal with the star-crossed quality of god/human relationships)
Kubera (uh, kind of)
Mahou Tsukai no Yome (though the romance element is very much in the background right now)

Try this one, which I loved to the max, "My Lovely Ghost Kana"

http://www.mangaupdates.com/series.html?id=7530

Iooking for manhua I don’t remember the name? It about actress got turn into a ghost then she wake up in his room she kiss him. The mc is idol making comeback with the help of ghost actress then someone steal her body then ghost female mc and male mc looking way to return ghost female back into her body.