I went through a terrible depression when my youngest left for college as well, and she was only 15 miles away! A variety of things got me through it, but one of the most practical was a book called "Letting Go." I believe it has been updated in the last several years and you can probably find it on such outlets as Amazon.
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When you give it away, it all comes back tenfold, they say.

http://www.thefellowshipofthedevastated.com

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