How do I get closure?

I was in a long-distance relationship for about seven months between August and February. It was my first love, and I had my heart broken. I was struggling with depression around the time of the breakup, and I sought professional help. My therapist needs me to write a farewell letter to my ex (but not send it, of course!) so I can get proper closure and not dwell on the breakup. How do I do that, and where do I start?