Often the leaver has had time to process this. They are probably (not always) done. And just "trying to be nice".
They probably seem ok/normal as they've lost the feelings and literally do see you as a friend. So. It's not really hard for her to go to centre parks with you.
For the leavee it's the opposite. You might be shocked, living in limbo and full of anxiety "can we make it work?", "do I have a chance to turn it around?".
Mostly the answer is no, and the person leaver isn't being honest and is subconsciously trying to be "nice" but it's worse. Because it's dragging it out.
It isn't always like this.. But it often is.
It might be no ones fault, you might both not have talked about issues when they arise (common).
I hope there is a chance for you to fix it. But often "I want a divorce" has been thought about long and hard but no talking has been done.
Sorry to hear this.