Heya there, Leon!
Unfortunately, there's no official guide on how to get all the endings - maybe that's something we need to fix.
However, you can check this thread (https://www.reddit.com/r/BLgame/comments/jo87in/comment/gcnl3fn/) to find how to get some endings.
Also, I'm copy/pasting a complete walkthrough for how to get one of the endings you're missing. Definitely scroll down through the comments, as some users have asked how to get specific endings themselves.
If you're still not be able to get them all, let us know.
(Spoilers)
The ending you're missing is "Breaking Free".
To get this one you need to be sweet and kind to Elijah until you get the scene where Isaac's high school friends invite him out and Elijah doesn't want him to go.
After that, you need to pick choices that are unpleasant towards Elijah: don't let him manipulate Isaac and rebel against him any chance you get.
If this tip still feels obtuse, feel free to follow the choices below:
* "Ask him to put a bag on his knees"
* "It's on me"
* "Caress his hair"
* "...to be his"
* "You <3"
* "You don't own me anything."
* "Come on, you deserve a break"
* "That's kind of sad!"
* "...to show him my love"
* "I have to match you."
* "It's not whatever"
* "Calm down, Don Juan!"
* "I know I can trust you."
* "Are you asking me to cancel?"
* "That's not what I said."
* "That's not the point."
* "That's not love"
* "Or you could quit yours"
* "No way!"
* "Elijah and I are quite busy"
* "Watch out for the quite ones"
* "Hussain?"
* "You could have told me if you didn't enjoy it."
* "A bit awkward, right?"
* "It's awkward"
* "You'll eat it raw next time."
* "I'm upset"
* "I'm not falling for that one."
* "Elijah asked me to"
* "No"
* "I shouldn't, should I?"
* "You raped me"
* "You know what you did"
* "Yeah."
* "Like cyanide"
* "Why won't he mind his own business?"
* "It was traumatic"
* "It's not about you"
* "You asked me to stop working."
* "What's his problem"
* "No"
* "Because that's what I deserve."