Ex boyfriend moving on help

My ex and I were together for a total of 5 months before we broke up in April of 2017. And now its been 4 months broken up from him. And he and I both live far away from each other on Oahu Hawaii..and I need help coping with the idea that he might get another girlfriend in college, since he graduated recently...Im a junior in high school. And I dont want to be around when/if he gets married one day. He and I used to tell each other, "I want to marry you one day". And "i will love you forever, I want to be with you forever." So any tips from anyone on how to start looking for someone else right now? I feel like anywhere I go...automatically no guys are attracted to me at all. And it makes it 20x's harder.