Heya everyone.
I just remembered this specific scene, but can't remember the title of the manga anymore. I'm not even sure if it was tagged as Yaoi, Shounen Ai, or both?

The setting is a shy uke (I think black-haired and quite feminine), who is in love with his childhood friend/senpai/whatever, but never tells him, because he doesn't expect any success from the confession. In stead he chooses an unknown seme (older, more mature) for some relief, but in the end actually falls in love with him.

The specific scene I'm mentioning is towards the end, where a bit earlier the uke was brave enough to tell the former-love interest that he had been in love with him, which the addressed didn't really get as a confession. It only dawns on the oldLI when he, the uke, and the new love interest all three sit in a bar/restaurant. There, something the newLI says makes the realization pop in the oldLI, who goes like O_O .... ? ... And the newLI just puts his finger to his lips to shush him from mentioning anything.

The description is probably Hella crazy... But that's just what I remembered, while trying to put in as much detail as possible :'D

Just the way it is, after three days of brain wrecking and hours upon hours of searching, I found it myself, quite shortly after making this post:
Bokura ga Koi wo Ushinau Riyuu
Is the six chapters long, completed series I was looking for ?