Barbara chose to escape and left Gordon. Barbara degenerated and became the same as before, taking drugs and drinking.
Barbara's blackening process
Barbara has a saying that I think is particularly good: "Let me share the other half for you".
Gordon didn't tell Barbara at first because he didn't want her to get involved in the maze of darkness and crime, and also to protect Barbara.
Then penguins appeared, and the story was exposed. When Barbara returned home, she was threatened to experience the dark corners of Gotham. Then she became mentally unstable, afraid and insecure. To put it bluntly, she is not strong enough to share Gordon's other half.
Gordon was too busy investigating the case to notice that Barbara was very wrong. Then Barbara ran away and slept with that woman named "Green" Gordon, and it was dark.