I've got to admit, I'm not sure there is a single true ending for the 'frog princess' story. It's one of those folktales that's been told a hundred ways. The most common version I know, the one I read as a kid, ends with the princess throwing the frog against the wall in frustration, which breaks the spell and turns him back into a prince. They get married. But I recently read an older, grimmer variant where she has to cut off his head with a sword! That felt more like the original 'true' ending, honestly—less romantic, more brutal fairy-tale logic.
Modern retellings smooth all that out. I read a YA novel last year, 'The Frog Princess' by E.D. Baker, which spun it differently; the princess herself gets turned into a frog, and the 'true ending' is about them working together to break both curses. It's cute, but it feels like a completely new story. So I guess the 'true' ending depends on which tradition you're pulling from—the Grimm's brutality, Perrault's slightly softened version, or a contemporary author's twist.