Lyrics and notes: Here are the words to the song (the lyrics) with some explanations.

Now and then I think of when we were together, [now and then = occasionally, sometimes]
like when you said you felt so happy you could die [like when = for example]

I told myself that you were right for me, [when you tell yourself something, you think it without speaking. We often use this expression for a thought that we later realize is wrong or misleading.]
[to be right for me = to be a good person to love]

But I felt so lonely in your company. [in your company=when I was with you]

But that was love and it's an ache I still remember. [ache=hurt, wound]

You can get addicted to a certain kind of sadness [to get addicted = to become dependent on, similar to a drug dependence.]

Like resignation to the end, always the end [resignation to=willingness to accept something that isn't positive]

So when we found that we could not make sense [we could not make sense = a good relationship was not possible]

Well, you said that we would still be friends, but I'll admit that I was glad it was over [to admit = to say that something is true; it was over = the relationship was finished]

But you didn't have to cut me off [cut me off = stop communication]

Make out like it never happened and that we were nothing [make out = pretend, act as if; we were nothing = our relationship was nothing]

And I don't even need your love

But you treat me like a stranger and that feels so rough [so rough=so bad, so hurtful]

No, you didn't have to stoop so low [ to stoop low is to behave badly]

Have your friends collect your records [to have your friends collect your records = to send her friends to pick up her recorded music from his home]

and then change your number [to change your number = to change a telephone number so that he cannot call her]

I guess that I don't need that though [that refers to communication or connection with the ex-lover]

Now you're just somebody that I used to know

(Kimbra)

Now and then, I think of all the times you screwed me over [to screw someone over = to hurt someone or to treat someone badly. It is not a polite expression.]

But had me believing it was always something that I'd done [her lover made her believe that, when he hurt her, it was her fault.]
[had me believing is not standard English; it would be more correct to say, "you made me believe"]

But I don't wanna live that way, [wanna=want to]

reading into every word you say [to read into someone's words = to find meaning beyond the words]

You said that you could let it go, [to let it go = to stop caring about something]

I wouldn't catch you hung up on somebody that you used to know [hung up on somebody = thinking or worrying about somebody in an excessive way]