Goodbye song:
Good Riddance (Time Of Your Life)
- Green Day

Listen to this farewell song:

This acoustic rock ballad, with contemplative lyrics, was originally written as a break-up song, but was famously used in the last Seinfeld episode in 1998. The lyrics are not specific to a romantic break-up, and can perfectly capture the feel of a bittersweet end of a journey.

Other songs like this:
Best Days - Graham Colton
Slow guitar folky ballad - about making the most of now, because "here come the rest of our lives".

You might like:
We'll Meet Again by Vera Lynn
Indulge in some wartime nostalgia with this classic song by Vera Lynn. 'We'll Meet Again' is a famous song of the Second World War era, and its optimistic tone a resonant comfort for soldiers and their families and sweethearts who were separated by the war. It's a perfect song for masking a heartbreak with a smile.

It's So Hard To Say Goodbye To Yesterday by Boyz II Men
Very soulful a capella R&B song, of which the original version was written by husband-and-wife songwriting team Freddie Perren and Christine Yarian for the 1975 movie "Cooley High". This version by Boyz II Men was more successful.

Anchors Aweigh by Bouncing Souls
This song has been described as an energetic, stirring song, reminiscent of a sea shanty - with a punk sound. The words are perfect for a farewell:
"Anchors Aweigh my friends,
I'll see you another day
I'm going away,
Experiences have to come
And the past has got to go".