Yes, all kidding aside, pretend she's imaginary. As in, don't let her alter your goals and interests. She is your 'ex,' meaning it's over.
I know the hurt won't readily go away - I'm sorry. But, in essence, she is not a factor any longer. I guess I would try to deal with the hurt in whatever way works for you. This is hard. I've been so hurt, in other ways, but this is separate from the incident (or ex, in your case). If that makes sense. I hope you feel better soon. 'They' (whoever that is!) say that 'it' gets better with time and distance. This is why you should pretend she's imaginary. If she chooses to engage you in the hallway, for instance, you could just nod, or say an innocuous, 'hi.' As in, neutral. And avoid her like the plague.
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The ones who say “You can’t” and “You won’t” are probably the ones scared that you will. - Unknown