Why do narcissists move on so quickly from a relationship after it has ended?

My ex-fiancé did this to me and within one month after splitting up she joined a dating website (I know this through a friend who told me.)

She even wrote on her dating profile that she has been 'badly treated' in her last relationships and was looking for someone who would treat her well.

Six months on and I'm still not feeling myself after the way she treated me and ended things so abruptly. I feel 'damaged' by her actions.