Sansheng, Wangchuan Wu Shang- Around chapter 6-7 the second life starts which is the ML is in love with his dead master who looked like the past FML. The novel I cant remember keeps merging with this premise. One key factor I remember is that the Ml had a secret place where he had pictures of someone he like(not mc) and later when enemies invaded one went to that cave and saw pictures/painting of the Mc( got over first love and started liking MC). Also i think(not sure ) the person who entered the cave was the person who ML liked first(before the MC). The Mc knew of the ML liking someone and they never thought that the Ml would like them back? I keep merging it with the Sansheng, Wangchuan Wu Shang story so i cant be more clear. I think the story included world hopping or system related and i also think that the setting might have been cultivation or something like that( for that specific world). this is because in the cave there where hidden cultivating book or power, jewel....idk(something important was there which people wanted to steal). Oh not sure sure if its a BL . also as i am think is it even a novel? is it a webtoon, manga, manwha?
@Vierne is correct. That's the 2nd arc of Holding Onto My Man. (Ch. 16-25) It's an ancient fantasy cultivation world. ML is a sect leader, white moonlight is his disciple, MC is another random outer (?) disciple who turns into his bodyguard... White Moonlight betrays ML for his own lover and wants to steal a treasure.. All three of them end up in the secret cave...