This seems to be an issue that happens every so often. Does the option "STAY" pop up for you in the forest when you make your branching choice?
Viewing post in Kokoro's Doki-Doki Sabbatical comments
You're welcome! Lol.
There's actually two ways to get the secret ending. If you can't get the all-clear end, you can also answer questions in a specific sequence to unlock it.
Assuming you've gotten all the other endings, you should be able to re-start and have the stay option pop up. Otherwise, it means you may not have answered some of the prompts right to unlock it the other way. If you're still having trouble after starting from the beginning, let me know and I can try to find a way to get you a guide.