You need to find the knife but not the other thing in the shelf, and when it comes to the trap you need to lose and get stuck in it the first time.
So I'm doing those steps, but now I looked at the in game guide and at some point you need 75 love pts to get that ending, and with the options from the guide love goes down often. I tried to figure it out myself but no matter what I do love always end up being 70 at most... I'm really sorry for being a bother-