Should I move on without him? How do I do that?

Nine years ago I rekindle an old high school romance. We love each other and have been on again off again for the past nine years. This time on our off again he is seeing someone new. This really shook me to the core. I really believed we would eventually be together forever. I told him how I feel. I love him and want us back forever. He says he thinks about me and us all the time but ran directly back to her. They've only known each other for 3 weeks. What should I do?