An amazing a capella version of Somebody That I Used To Know, originally by Gotye.

Now and then I think of when we were together
Like when you said you felt so happy you could die
Told myself that you were right for me
But felt so lonely in your company
But that was love and it's an ache I still remember

You can get addicted to a certain kind of sadness
Like resignation to the end
Always the end
But when we found that we could not make sense
Well you said that we would still be friends
But I'll admit that I was glad it was over

But you didn't have to cut me off
Make out like it never happened
And that we were nothing
And I don't even need your love
But you treat me like a stranger
And that feels so rough
No, you didn't have to stoop so low
Have your friends collect your records
And then change your number
I guess that I don't need that though
Now you're just somebody that I used to know
Now you're just somebody that I used to know
Now you're just somebody that I used to know

Now and then I think of all the times you screwed me over
Wanting me believing it was always something that I'd done
But I don't wanna live that way
Reading into every word you say
You said that you could let it go
And I wouldn't catch you hung up on somebody that you used to know...

But you didn't have to cut me off
Make out like it never happened
And that we were nothing
I don't even need your love
But you treat me like a stranger
And that feels so rough
No, you didn't have to stoop so low
Have your friends collect your records
And then change your number
I guess that I don't need that though
Now you're just somebody that I used to know
Somebody… I used to know
Now you're just somebody that I used to know
Somebody… I used to know
Now you're just somebody that I used to know

I used to know
That I used to know
I used to know
Somebody...

NOW AND THEN= Sometimes.

YOU FELT SO HAPPY YOU COULD DIE= A common way of expressing that you feel immensely happy.

LONELY= If you feel lonely, you feel alone, like nobody around can understand you or love you. You may be with a lot of people around and still feel lonely. To be ALONE is the physical fact of having nobody around. "Lonely" is the way you feel, "alone" is how you are, both words are completely different.

ACHE= /eɪk/ pain, suffering. (we also use it in "headache, stomacache," etc)

LIKE= Such as, for example.

WE COULD NOT MAKE SENSE= Our relationship couldn't continue.
If something makes sense, it's logical, reasonable, understandable. If something doesn't make any sense, there's no logic in it, it's impossible to understand, it can't work.

IT WAS OVER= It finished (to be over = to finish)

CUT ME OFF= Separate me completely from your life (radically and suddenly in this case).

MAKE OUT LIKE= Pretend, act as if...

ROUGH= /rʌf/ hard, difficult; painful.

STOOP= To bend forward and down, as when you want to pick up something from the ground without bending your knees. But figuratively we can use it meaning to abase, to lower in rank, prestige, or esteem, to degrade, to become a worse person (and this is the sense they use here in this song). By saying "stoop so low" they say that you degraded a real lot (and they give an example: she didn't even want to go back to his house to collect her things, she just sent her friends to get her things so as not to see him anymore).

HAVE YOUR FRIENDS COLLECT...= If you have somebody do something, they do it for you (because you ask them to).
- I had my mother pick my kids out of school because I had to stay with a customer.

CHANGE YOUR NUMBER= She changed her telephone number so he wouldn't be able to contact her anymore.

I GUESS= (coll.) I suppose.

THOUGH= /ðəʊ/ Nevertheless (but used at the end of a sentence):
She's Italian. Nevertheless, she can speak perfect Spanish =  She's Italian. She can speak perfect Spanish though.

I USED TO KNOW= The verb "used to" marks a contrast between present and past, it expresses an action that was true in the past, but not anymore.
- When I was a kid, I loved playing basket (maybe I still do)
- When I was a kid I used to like playing basket (this means that now I don't like it)

SCREWED ME OVER= (vulgar) Deliberately putting someone in an unfavourable unpleasant situation. Hurting someone in some way.

READING INTO EVERY WORD YOU SAY= Analysing carefully all you say because I think there is a double meaning in your words, like when you say "do you need any help?" and they say "no, thanks, I'm fine", and you read into it something like "you're disturbing me, go away, I hate you". Yeah, you can easily get neurotic when you constantly read into what other people say.

LET IT GO= Forget about it (after a traumatic situation)

HUNG UP ON= (slang) Obssessed with.

Breaking up is hard to do, especially if they just suddenly leave you by acting like you mean nothing to them and completely disappearing from your life, radically, completely.

A few days ago I was the love of your life, and suddenly, I became just somebody that you used to know. That feels so rough!

This is a cover of a song by Gotye. In the original song, it's only a man and a woman singing, so all the parts are sung by Gotye except the woman's part, who is sung by Kimbra. So we have to take this song as a conversation:

The man complains about the way she broke up. She completely cut off and disappeared and now treats him like she doesn't know him. And then she complains about how hard it was to live with him, and explained that when they broke up, he told her that he could just "let it go" (forget about it) no problem, so she's acting accordingly. But then the man replies and says once again that her reaction was too harsh, too radical, and that hurts.

So who's right? Probably both of them, or rather none of them, as usual.