It's So Hard To Say Goodbye To Yesterday - G.C. Cameron (from the 'Cooley High' soundtrack).
Go see the movie and then listen to the song and more than likely you will cry.
(A 30-second sample: http://www.cdnow.com/cgi-bin/mserver/SID=500005706/pagename=/RP/SHARE/soundclip.html/UPC=0121576762/disc=01/track=21/source=wmf )
Boys II Men did a remake, also.
The lyrics:
How do I...say goodbye...to what we had?
The good times that made us laugh...outweigh the bad.
I thought we'd get...to see forever...but forever has gone away.
It's so hard...to say goodbye...to yesterday.
.......
I don't know...where this road...is going to lead to.
All I know...is where we've been...and WHAT we've been through.
If it gets...me to tomorrow....I know it's worth all the pain.
It's so hard...to say goodbye...to yesterday.
.......
And I'll take...with me the memories...to be my sunshine after the rain.
It's so hard...to say goodbye...to yesterday.