The official definition is to be intensely aware of what you’re sensing and feeling in the moment, without interpretation or judgement. With regards to the ending of a pathological relationship I think you can add something more helpful to that.
For example: When you are obsessing about the narcissist or his new target, you stop, become aware of your feelings and re-route them. Think of the feelings like clouds in the sky. They aren’t permanent, they are simply drifting across your vision. And then you re-route your thinking as well. Instead of obsessing you say to yourself, ‘I am enough.’ That piece of wisdom, the ‘I am enough,’ comes from Marisa Peer.