I went through exactly this in 2006. It was really really painful and I still miss my friends (husband & wife) terribly. They were the only people I would have classed as close friends then, and they still are.
I write long emails to them, which they sometimes answer and sometimes don't.
I visit them in their new country at least once a year. I get to meet the husband maybe once a year elsewhere (we have the same interests and our paths cross occasionally).
But it's not the same.
I still love them dearly and miss them every day.
I have no advice to make it easier.
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Female. Dx ASD in 2011 @ Age 38. Also Dx BPD