Im an adult and I like manga, but I am having a hard time finding new マンガ that attracts andkeeps my interests.

berserk sounds like something you'll like its set in midevil times has magic monsters main character is not a good guy as for the pretty boy thing griffith the storys villen should be fine ill put a pic so あなた deside yourself im a sraight man so i wouldnt relly know any thing on a dudes looks and its an adult マンガ not hentai just relly violent

Hmmm... Higurashi: When They Cry is a suspenseful, スーパーナチュラル story. There is no hot guy with long hair and it is in the present, but it's very suspenseful. Beserk is definitely up your ally (and I can't think of any other マンガ that incorporates all of them either off the 上, ページのトップへ of my head). Hell Girl is pretty dark and has スーパーナチュラル stuff in it. Pet ショップ of Horrors is a dark マンガ too and definitely shows the dark side of people. It's スーパーナチュラル but set in modern times, and I think the ショップ owner is pretty hot...

(image is hell girl)

hmmm well some of these are aimed at teenagers but they may appeal
Moon phase
Vampire knight( this is aimed at 13+ but i think the age ranger goes from 13-32)
Chobits( it's okay but there's lot of explecite material yeah i know your an adult but some ppl don't like that stuff)
loveless( やおい lots of it if あなた don't like やおい then don't read it)
thats all i can think of

There's a website that あなた can choose your genre's and find what your お気に入り Mangaka's mangas' are. あなた can also go on マンガ 検索 to find your お気に入り manga.
The website is:
link

On マンガ Fox:

White Guardian (mangaka Duo Brand almost anything によって her) Excellent.

I recommend the following as アニメ (and I know あなた asked for manga...but あなた may like the マンガ versions:

A Cicada in Winter (Fuyu no Semi): Historical, Long hair, やおい 愛 story, dark.

Koutetsu Sangokushi - archaic, supernatural, dark, warriors, magic, fantasy. I hate machinery too and there is a teensy bit in this, but I highly recommend it anyway. Long hair.

Mirage of Blaze: dark, fantasy, supernatural, part historical(hot guys but little long hair)

Kya Kara Maou または Kya Kara Maoh: hot guys, some hot guys with long hair, fantasy, adventure, magic, warriors - archaic setting. Grows darker.

黒執事 - historical magical fantasy, dark, supernatural, but no long hair.

If あなた look on
link
you'll find a huge amount of interesting やおい mangas - there are a few historical ファンタジー stories with delicious guys and long hair ...and swords.... and あなた can look them up on Mangafox または Google to read.

If あなた really like Kenshin but read it all and want もっと見る あなた should read Samurai Deeper Kyo.

Its not exactly the same as it seems to use is similarities to fool あなた with one of its major plot twists.

And I think あなた would like Demon Diary as it is about a kind and gentle boy who becomes the successor of a Demon Lord.

The person who shows him the ropes (almost completley unsuccesfully)
has long hair and is basicly a 愛 intrest of the lad.
Its a very good all-around マンガ that can be very funny and very serious.

Image from Demon Diary:

do あなた read manhwa as well? Because I know one it's called crazy girl shin bia ... historical, and guys with long hair *not to mention hot). BUT the heroine is kind of annoying (for me).. well she's pretty not cute and very gangster like .. no manners and such but not dark.

how about FullMetal Alchemist? its an awesome マンガ in my opinion.....
または the マンガ Hell girl? あなた can find it on manga.animea.net i think....
または Tsubasa Chronicles?
または xxxHOLIC
または Stray dog? its pretty good but it ended too quikly.........
または um.... ちび Vampire? (im not sure if youll like it though..)

maybe chobits.