How are you supposed to feel when your ex treats his new girlfriend better than he treated you?

You're not alone. Others have been through what you have. Personally I've been through a ridiculously bad string of girls who did nothing but take advantage of my generosity and then cheated. It's an awful, terrible feeling when you do everything you can to make the other person happy, but they continue to treat you like garbage. The only way you can cope with it, is to have hope, hope for the future. You just be optimistic that you'll find that one person who will realize the good thing they have and will do anything to keep it.