Scope out other guys! Flirt. Write him a letter that you'll never send. But always imagine scenarios of you sending it to him. Or imagine him seeing you years later, you being happy and successful, and he'll feel so stupid for letting you go.
Also, despite all of the above is based on revenge and spitefulness, do your best to forgive him over time. Nothing lasts forever. We all know that, you'll find someone better.
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