For no particularly good reason, in no particular order, and probably not particularly comprehensive, here's a list of "letting-go" songs that I have found particularly moving over the years.
Kite - U2 (All you can't leave behind)
Walk like a man - Bruce Springsteen (Tunnel of Love)
The Drugs don't work - The Verve (Urban Hymns)
September Blue - Chris Rea (Dancing with Strangers)
Mandolin Rain - Bruce Hornsby & the Range (The Way it is)
The River Runs Low - Bruce Hornsby & the Range (The Way it is)
One Tree Hill - U2 (Joshua Tree)
If you are feeling maudlin, make them into a playlist and weep for a week.