(Comment contains spoilers)
Let me immediately begin by thanking you so much for your determination and patience! It means a lot when I see someone trying to see the secret ending. You've put in more than enough time than I expected and you deserve clarity! I must admit I think I've made this secret ending too hard/convoluted to get. Multiple oversights on my part. I haven't heard of anyone getting it in comments or videos, so I'll give you some hints and hopefully others will benefit from it too.
Based on what you've figured out, you're not far from getting the secret ending. From my experience with puzzles, being told the answer is never as rewarding as figuring it out yourself, so this comment will not spoil it entirely (but will get close). Feel free to stop reading if you get any ideas. But I also understand that's not how everyone likes to play games, so if you want the answer straight up please let me know and I will be happy to provide in further comments! I spent a solid bit of time on the secret ending so I hope it's worth it.
To get the secret ending, there is a certain action you have to perform at a certain time. The advice has two important parts to use.
1. The first is the group of 4 underlined words, forming an idiom. In the entire game, there is only ONE other time that idiom is used. You can only get the secret ending when the paragraph containing that idiom is the one you're currently "reading". That is to say, the second it shows up, wait for any automatic text to finish and do NOT press space! If you press space, the next paragraph will start typing and the chance is gone. I know ... very unforgiving. (This is what "only in that very moment" tries to imply).
Oversight by me: The idiom only shows up the second time if the game knows you've received the advice during THAT playthrough. Originally it would show on every playthrough, but that left open the possibility for people to accidentally stumble upon the secret ending without experiencing the bad ending first (which would narratively make no sense). So, it only shows if you have the advice, which you can only get after the bad ending. Not the best idea in retrospect because people might not realise this and without a save system the game takes a long time to play.
That leads me to my next oversight: the idiom doesn't show up at the most convenient time - it's not at the beginning of the game, nor in the middle ... sorry. It's not during the bad ending either, so never click "It's no use. There's no way out.". You also have to have given the old lady her painting.
2. The second important part is the action you have to perform. Hitting the X to close the window isn't the answer: Due to the way the game is currently programmed, no information is retained when the game window closes (hence why there is no save system). When the game is reopened, it has no knowledge of what you did on previous playthroughs.
There will be an indication when you perform the right action - you should notice something different. If you stop at the right moment and don't hit space, you should eventually find it.
If only there was some functionality implemented into the game that seemed redundant. As if the entire game could be perfectly playable without it, but for some strange reason … it's there.
Hope this is enough and good luck! I'll respond if you want further hints or the answer. Use headphones to get the most of the secret ending. :)