welp, accidentaly the second ending I approached is the 10th ending... and I warn you it's the angstiest one after each bad endings from the three LI. Explaining this, I prefer to acknowledge there are a "bad" choice and a "good" choice for every heroine's reaction to LI.
(SPOILER - NOT SPOILER ALLERT!)
1. This instruction apply to every Love Interest you chose.
2. The 10th ending only could be achieved when the first two choice path is balance. It means : you have to get one "good" choice and one "bad" choice because....
a. if you have both "good" choices, then it will lead to Friendly ending or Romantic ending depends on your last choice ;)
b. if you have both "bad" choices, then it automatically ends your heroine to hell of Sad ending :')
3. then, at the last choice, chose intentionally the "bad" choice, so that the total answers you achieve are 2 "bad" choices and only 1 "good" choice.
4. Last, welcome the story of 10th Ending named "Lethe", and how irronically the real signification of that ending's title :'(
Hopefully it helps you :D