Like a previous poster mentioned, forgiveness and finding your own peace are a big part. The person who wronged you may never come back begging for forgiveness but forgive them anyway! Otherwise, you're walking around with hatred in your heart that can consume you.
Also, figure out what is was you were supposed to learn from that situation . All relationships, good or bad, have some sort of lesson attached to them. I find that taking this approach keeps me from being cynical and allows me to remain open to new relationships.