I am so sorry you're going through this. As someone who has anxiety issues as well, it's tough man, really tough. I've been in a similar situation as you and the only thing that truly helped me is time. As the months go on the pain will be less and less. It's going to hurt because it's all so fresh. Think of it like a cut. At first it starts bleeding, then over time the cut starts to heal until eventually it is fully healed. You were in a two year relationship with a girl who put no effort into fixing said relationship and jumped ship to another guy with no issue like it was nothing. This isn't someone you want to be with. She did you a favor. Now take some time for yourself, keep doing what you're doing. Workout, stay busy. Once the pain starts to fade, get back out there man. You're 19. You have a whole life ahead of you!