So I wasted an hour figuring out the song, and by the time I did, I saw this: 
http://rebeccasugar.tumblr.com/post/33762090124/remember-you-demo-and-chords-c-f-fm

I have never been so proud and so pissed at the same time. Anyways, please enjoy 
my version of the song.

Note: E7/G goes like this:
e|0
B|0
G|1
D|0
A|2
E|3
But you can play E7 instead of E7/G, like in the official version (http://
rebeccasugar.tumblr.com/post/33762090124/remember-you-demo-and-chords-c-f-fm).

Remember You by Rebecca Sugar, performed by Olivia Olson (Marceline) and Tom Kenny (Ice King)

Intro: C F Fm (2x)

C                     Fm
Marceline, is it just you and me in the 
                        C
wreckage of the world?
             F            Fm
That must be so confusing for a 
            C
young girl.
                  F                Fm             C
And I know you're going to need me here with you.
        F                       Fm                        E7/G
But I'm losing myself, and I'm afraid you're gonna lose me too.
                     D                        G
This magic keeps me alive, but it's making me crazy,
              C                            F
And I need to save you, but who's going to save me?
          Fm                    C
Please forgive me for whatever I do,
               E7/G
When I don't remember you.

C F Fm (2x)

C                Fm         F             C
Marceline, I can feel myself slipping away.
          F              Fm           C
I can't remember what it made me say.
        F             Fm            C
But I remember that I saw you frown.
           F             Fm             E7/G
I swear it wasn't me, it was the crown.
                     D                        G
This magic keeps me alive, but it's making me crazy,
              C                            F
And I need to save you, but who's going to save me?
          Fm                    C
Please forgive me for whatever I do,
               E7/G
When I don't remember you.
          F                         Fm
Please forgive me for whatever I do,
                 C
When I don't remember you.
      F     Fm       C
La ah da da da da da da,
   F     Fm    C
Da da da da da da,
   F     Fm    C
Da da da da da da