This game needs a better walkthrough, like the kind written by
Zoey Raven because the current one is terrible.
There seem to be multiple different endings in this game (Christine ending, Jess ending, Christine and Jess ending, Christine leaves ending, ntr ending, etc, etc), but the current walkthrough guides towards just two of these and it's unclear exactly what decisions and what points you need to trigger the specific ending you want.
I also agree that the ending feels a little rushed because it feels like there was still more to be done, like how having sex with Monica was a perfect segway into having her accept your relationship with Christine because she would hardly be one to judge since she's harboured feelings for the MC for years and then had sex with him. It's also rather sad the way things go with Monica because throughout the story she's seen as something of a bad guy but then you find out that's she's not all that bad, and she's fucking hot. Not getting a lot of time with her is real letdown.
Thene there's the whole thing with Jess because I was pursuing her on my playthrough and in the scene right after the nightclub where she has a hangover, she's thinking about her feelings for the MC and what they mean to each other, but then it's forgotten about and there's no proper resolution for it by the end. The lack of a proper poly/harem relationship where all parties are together and in love was another letdown of an otherwise great game.