Barns Courtney-Hard to be alone (8D) Use Headphones 🎧🎧

Lyrics:
I could see the city written in your face
Freckles spread like constellations all over the place
Darling, I walked you to the station
Shooting with my heart on my sleeve
Now I'm staring at your photograph
And hoping you ain't made a fool of me
So if you're struggling to go to sleep tonight
When you're on your own beneath the lonely lights
Oh, darling, if I ever cross your mind
Won't you let me know?
Oh, it's hard
Oh, it's hard to be alone
Oh, it's hard
Oh, it's hard to be alone
Moved to the big city and called it home
Oh, what a crowded place to live alone
She was a choir girl when we were young
What have we done, what have we done
So if you're struggling to go to sleep tonight
When you're on your own beneath the lonely lights
Oh, darling, if I ever cross your mind
Won't you let me know?
Oh, it's hard
Oh, it's hard to be alone
Oh, it's hard
Oh, it's hard to be alone

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what does 8D mean?