How do I win my ex back when he recently started seeing someone else?

Why would you want him back? More info please.

You don't. There were reasons he left. There were problems you both didn't or couldn't work out. Instead of taking the proper time to learn from the mistakes of the past relationship and heal, he jumped ship and moved directly to the next person. This is not a healthy way to go as his issues never get addressed. He is searching for something you didn't provide him by quickly moving on.

The person who initiates the break-up would have to make the first move to reconcile anything. Even then it may not be worth it. It would be best for you to learn from this experience and move forward. He already has...

It's not game. He's not a prize that you win.

If he doesn't feel the same way about you, then you can't force him.

He had a change of heart, so accept and move on.