The same is happening to me rn but I was forced from my old barn and never got to give me horse a proper goodbye :( I recommend just trying to do more research on where you might be going and look at the positives about the whole situation.
Hi, I know how you feel it is upsetting but you will meet lots of new horses too and if you don't live to far away you could go visit him .
Be sure to spend plenty of time with him before you leave. Lots of hugs, kisses and treats will make the goodbye a bit easier. Hope that helps!