If that doesn’t work, you can discuss something that makes you angry–say a political stance you both agree on–or some disturbing news. Transition from these topics at least twice by telling your partner that the sad topic is too serious for the moment.
After a happier topic, you can transition to another sad topic again. Try: How to talk about a past relationship with your partner] What is the purpose of the steps outlined above?
Lastly, you can grab her attention by starting a conversation, then bowing out before it gets too deep.
After a while, you can resume the conversation and let it last a bit longer.
Open up about something random that most people get sad about, like a traumatic breakup.