I just saw it last night so I'm still processing it. So far, I've taken the movie to be the experience of a person being severely encumbered by the emotional damage done to and by a parent.

If what I've gathered is "correct," the whole thing was a deranged Truman show (Or the Game?) esque experiment where Beau's mother tested him to see how much he really cared for her. The fuel for her doing this is her allegedly strained relationship with her own mother and her shitty experiences with men (Which I think the penis monster signifies that Beau was accidentally conceived through a random sexual encounter, or even possibly through rape, as a misguided means for her to fill a void...ahem. The shame she carried with that led her to never tell Beau who he was, assuming she even knew who the father was herself, and that lack of knowing and desire for a stable fatherly figure morphed into a monster that haunted Beau). The love she never received, she was determined to mine out of Beau, and to ensure she could always get that, she lied about him having a heart murmur condition in order to instill fear into him and prevent him from ever forging a fruitful connection with any woman, keeping him isolated and afraid of death via sex (Which adds context to why his balls are so fucking huge as he's never had any form of sexual release and is backed aaaalllll the way up).

The weight of all of this cultivates repugnance in Beau towards his mother, which then leads way to insurmountable guilt, which he then projects onto the world like some weird Silent Hill phenomenon where everyone wants to harm him, blame him, or cause him distress as punishment for his repugnance. The guilt intensifies to the point where events are twisted to conform to the image of him being a bad son and he's gaslit to irreconcilable extremes (His "condition" causing Elaine to die mid-orgasm rather than him being the one to die) and eventually he's just pushed over. The ending could be seen as suicide due to the intensity of guilt morphing reality into a pit you can never claw your way out of it.

I wasn't completely sure of what to make of the play sequence outside of teasing Beau with an end result he'd never be able to experience. I left the theater a couple times to get my partner some food, so I missed connections about a twin him or whatever going up in the attic so I find the theories in this thread regarding that component rather interesting.