Ed Sheeran just posted the live version of I'm A Mess (link below) and here is how
he plays it.

https://www.youtube.com/watch?v=-t2CR9qZRj0

First off in this version the tuning is down a half step so instead of tuning to
E B G D A E you need to be tuned down a half step to Eb Ab Db Gb Bb Eb.
(There are other versions of this song he does live and it's tuned down a whole step)

Then there's the chords. Because we're tuned half a step down the chords below are
the shapes of the chords, they're not the sounds. So the E is an E shape not an E
sound. When we're tuned down a half step and E shape will actually be an Eb.

Where the x is on the B and C#m Ed rests his middle finger on the bottom string to
mute it.

     E    Asus    B     C#m    F#m    A    E2
e|---0-----0------0------0------x-----x-----0---|
B|---0-----0------0------0------x-----x-----0---|
G|---1-----2------4------6------x-----x-----9---|
D|---2-----2------4------6------4-----7-----9---|
A|---2-----0------2------4------4-----7-----7---|
E|---0-----0------x------x------2-----5-----x---|

          C#m
Ooh I'm a mess right now
       B
Inside out
                E
Searching for a sweet surrender
    Asus            E
But this is not the end

        C#m
I can't work it out
  B
How?
                  E
Going through the motions
Asus          B
Going through us

    Asus                             E
And oh I've known it for the longest time
              B            Asus
And all of my hopes all my words 
                            E
Are all over written on the signs
                 B               Asus
But you're on my road walking me home

Home, home


C#m                         B
See the flames inside my eyes
            Asus                     E     B
It burns so bright I wanna feel your love, ohoh
C#m                   B
Easy baby maybe I'm a liar
          Asus                  E
But for tonight I wanna fall in love
             F#m         A
And put your faith in my stomach


         C#m
I messed up this time
          B
Late last night
            E
Drinking to suppress devotion
     Asus         E       B
With fingers intertwined

        C#m
I can't shake this feeling 
 B
Now
                        E
We're going through the motions
Asus         B         
Hopin' you'd stop

    Asus                               
And oh I've only caused you pain
  E                  B           
I know but all of my words
              Asus 
Will always below
                        E         
Although all the lie we spoke
                  B               Asus
When you're on my road walking me home
            
Home, home, home


C#m                         B
See the flames inside my eyes
            Asus                     E     B
It burns so bright I wanna feel your love, ohoh
C#m                   B
Easy baby maybe I'm a liar
          Asus                  E
But for tonight I wanna fall in love
             F#m         A
And put your faith in my stomach


    C#m E2  B        Asus E  B  C#m
And for how long I love   my lover
C#m E2  B         Asus E  B  C#m
For how long I love    my lover, now, now
C#m E2  B               Asus E  B  C#m       
For how long, long I love    my lover, now, now
C#m E2  B               Asus E  B  C#m       
For how long, long I love    my lover, now, now

C#m             E2 B          Asus E     B    C#m
And I feel love,     I feel it all over, now, now
C#m             E2 B          Asus E     B    C#m
And I feel love,     I feel it all over, now, now
C#m             E2 B          Asus E     B   C#m
And I feel love,     I feel it all over, now
C#m             E2 B          Asus E     B    C#m
And I feel love,     I feel it all over, now, now

C#m              E2  B               Asus E  B  C#m  
And I feel love, how long, long I love    my lover, now now
C#m E2  B               Asus E  B  C#m       
For how long, long I love    my lover, now, now
C#m E2  B               Asus E  B  C#m       
For how long, long I love    my lover, now, now
C#m E2  B               Asus E  B  C#m       
For how long, long I love    my lover