This is a really fun game, full of beauty. However, the conditions for its good ending are too harsh. It is recommended to add a guide for players to follow.
I made a mod that does exactly that: https://tanxui.itch.io/causality-oialt. Also, there's a written walkthrough made by the dev among the game downloads.
Getting the best ending with all the girls is quite difficult, yeah. And messing up hurts, since the game is so well made and now you get to play less of it.