Hmm... it doesn't sound like you're doing anything wrong >.<
I just did a quick speedrun on a clean Windows test copy cos it's been a while since I worked on this and my memory was a little fuzzy, so I figured I better double-check exactly how it works, haha.
Okay, so, as you say, you need to reach the bliss ending to unlock the extras, and you'll know when you've successfully passed that ending because a screen will come up that says it's the bliss ending. You'll also get a credits roll, and at the end of the credits, it should have a message that tells you you've unlocked the extras section + how to access it. After that, there's a short extra scene, and then it kicks you back to an alternate title screen.
Once on the title screen, you need to navigate to the top left corner of the screen and if you hover the mouse in the right place, a little image of a bunny should appear.
I took a quick screenshot to show the general area that you need to highlight and put blue marker on it :3
A small button that looks like this should pop up when you hover over the marked area.
If you click on the bunny button, it should take you to the extras :3
You can actually find and click on the button even when the extras are still locked, but it will just pop up with a message saying that the extras are unlocked by reaching a certain ending x3
I only hid it up there because I didn't want to make the other main options look cluttered on the screen, haha.
If you've done all of that and the extras still won't open for some reason, then I'm not really sure what would cause it to be broken on your end since I can get them to unlock on my copy >.< I'd need to somehow replicate everything exactly the way you're doing it in order to get to the bottom of why it won't unlock for you.
As a checklist, all I can say is:
Make sure you succeeded in all 5 (I think it was 5 anyhow x3) timed/quicktime presses during the final scene where you have to keep stroking Blythe. If you don't hit all 5 fast enough, it will default to a different ending (Torpor End) instead of the Bliss one.
Make sure you see the screen that says Bliss End on it.
Make sure you see the note in the credits roll that tells you the extras have been unlocked.
After all of that, when you go to click on the bunny button in the top left of the title screen, let me know if it allows you into the extras or if it comes up with a message telling you to reach a certain end in order to unlock them still.
It's possible if you're not playing a Windows version that there could be some weird Mac/Linux bug because I can't test anything on either of those T_T but I don't really see why it would work for me on Windows and not on other systems if the game functions on those systems in general.
There's a high chance I'll be unable to fix it for you if nothing above works though :(
If it comes down to it, I could always export you a separate copy of the game with the dev console active, then it would just be a case of typing in 1 short command into the console to unlock the extras!
But an easier option is I could just send the extras files over to you via Google Drive or something if you'd prefer :3
Thanks so much for playing the game by the way!! <3
(and sorry if none of what I typed is of any help >.<)