How do I act around my cheating ex?

My soon to be ex cheated on me. The deed included all the typical painful events that occur in such cases... she lied, deceived, manipulated etc. I have to break up with the next time I see her, probably tomorrow. It of course hurts a lot because she's been my friend for five years. I live abroad, and before this I counted her as one of the only friends I have in this country. My problem: We work together. I have to see her every single day. I don't think she'll feel any real regret. To be completely happy will be so fake and she'll see through it. She knows how much I love(d) her. To be sad in front of her will demean me and hurt my dignity even more. Is there a middle? How do I act to be fair to myself and while not giving her anymore than she's taken from me?