Hi guy, I'm looking for a manga with this pattern: a boy and a girl obviously like each other but they are not really an official couple (optional); somehow the girl has some sort of "dark" secret or ends up in the situation that can consider "cheating" and it drifts them apart. Some examples I know are Natsuki and Takumi in Initial D, Kyoichi and Chiharu in boys be (anime version), Yuuki and Futaba in Salad Days.
I don't mind shoujo or josei but I prefer to see it in shounen or seinen.

Though it doesn't exactly match with what you want, maybe you could try these:
Haru yo koi
Cat Street

Don't only look straight, look also to the sides, usually the essential isn't in front of you, but beside you.
-7

You can try
Peach Girl---kind of fits what you're asking, there are many hardships between Toji and Mo Mo... haven't finished the manga myself...but many things drift them apart.

Brainstorming 🙄 :
Eensy Weensy Monster - the heroine has a lively dark secret that keeps her from getting closer to the school prince.
Indigo Blue - Yuri - A novelist is secretly attracted to another woman, even though she has a boyfriend.
Goong - there is a love triangle, and at some moment it appears that she's plotting with her husband's rival 😲
Akuma to Love Song- love triangle and she has some "dark secrets"
Hot Blooded Woman - funny manhwa full of dark secrets and crazy romance.

Chisa x Pon is like that.

Also Suzuka to some degree, though I found it pretty annoying at times.

Thanks for all your great suggesstion. I'll try them all and see.