I joked with my girlfriend that we broke up, but later she took it seriously. It's no use explaining. How can I make him forgive me?

If she doesn't really want to break up with you, and your joke just gives her a legitimate reason.

Then he just wanted to scare you so that you wouldn't dare to joke about it in the future.

But you are also good, making fun of breaking up. Is it fun? Is this a joke?

Coax her well, and then promise her that she will never joke like this again. You already know the seriousness of your mistakes and problems.

Hold on for a few days. If I still don't forgive you, it is estimated that she will break up.