Most Helpful Guy
Don't, period. That's called playing games. He's gone and with somebody else. Accept it and move on. Pursuing this is being a borderline obsessive stalker. Don't let yourself go that direction.
Most Helpful Girl
You don't. Doing something like that will 99% of the time not make him regret choosing someone else over you. So in the end you'll still be alone, on top of looking and feeling like an idiot.
The best way to get redemption if you insist on getting that is showing him that you're fine, if not better, without him by your side.