I've found all chords by ear and synced them with the lyrics.
The outcome sounds quite accurate to me.
Hope you enjoy it! =)


        C/B       Gm       Bb=A#
      o|-0-|    o|-3-|    o|-1-|
      o|-1-|    o|-3-|    o|-3-|
      o|-0-|    o|-3-|    o|-3-|
      o|-2-|    o|-5-|    o|-3-|
      o|-2-|    o|-5-|    o|-1-|
      x|-0-|    o|-3-|    o|-1-|


[Capo 4]


INTRO: C C/B Am7 F (x2)


VERSE 1:

C                   C/B
 Can't believe you're packing your bags
Am7               F
 Trying so hard not to cry
C                      C/B
 Had the best time and now it's the worst time
Am7               F
 But we have to say goodbye


PRE-CHORUS 1:

G
 Don't promise that you're gonna write
       Dm
 Don't promise that you'll call
G                                      F
 Just promise that you won't forget we had it all


CHORUS:

                 C               Gm
 'Cause you were mine for the summer
        Bb               F
 Now we know it's nearly over
            C          Gm
 Feels like snow in September
       Bb            F
 But I always will remember
          C Gm       F     
 You were my- summer love
     C              C Gm       Bb
 You always will be my- summer love


VERSE 2:

C                 C/B
 Wish that we could be alone now
Am7                       F
 If we could find some place to hide
C                   C/B
 Make the last time just like the first time
Am7            F
 Push a button and rewind


PRE-CHORUS 2:

G
 Don't say the word that's on your lips
       Dm
 Don't look at me that way
G                                      F
 Just promise you'll remember when the sky is grey


CHORUS:

                 C               Gm
 'Cause you were mine for the summer
        Bb               F
 Now we know it's nearly over
            C          Gm
 Feels like snow in September
       Bb            F
 But I always will remember
          C Gm       F     
 You were my- summer love
     C              C Gm       Bb
 You always will be my- summer love


BRIDGE:

           G
 So please don't make this any harder
    Dm
 We can't take this any farther
     F                                   F
 And I know there's nothing that I wanna change, change


CHORUS:

                 C               Gm
 'Cause you were mine for the summer
        Bb               F
 Now we know it's nearly over
            C          Gm
 Feels like snow in September
       Bb            F
 But I always will remember
          C Gm       F     
 You were my- summer love (summer love)
     C              C Gm       F
 You always will be my- summer love (summer love)
     C              C Gm       F
 You always will be my- summer love (yeah)
     C              C Gm       Bb
 You always will be my- summer love