Hillary Clinton (Yana Paskova/For The Washington Post)

It has been 18 months since the world found out that Hillary Clinton exclusively used a private email address and server to conduct her official business as secretary of state. In those 18 months, Clinton has been, at turns, defensive, dismissive, mildly apologetic and just plain annoyed over repeated questions about her initial decision to set up the server.

She has repeatedly offered legalistic answers chock full of caveats and I'm-sorry-if-anyone-was-offendeds. What she has never done is simply and totally apologize for an error in judgment that has badly complicated her march to the White House. I've speculated that her unwillingness to do stems from the fact that Clinton personally views the whole issue as ridiculous so she just can't bring herself to offer up a full-throated apology.

Well, I was wrong. Because on Wednesday night in a call-in interview with CNN's Anderson Cooper, Clinton gave her best answer — by far — to a question about her email server and how it has affected how people view her. Here's the whole thing:

"I have been asked many, many questions in the past year about emails, and what I have learned is that when I try to explain what happened, it can sound like I am trying to excuse what I did," Clinton said. "And there are no excuses. I want people to know that the decision to have a single account was mine. I take responsibility for it. I apologize for it."

That is, quite literally, a pitch perfect apology/explanation. It shows genuine contrition, offers her zero outs or excuses and fully embraces her own responsibility and role in the whole thing.

Watching it live last night, the only thing I was left to wonder was why she hadn't said/done this earlier. And I still don't know that answer. My guess is that there is a reflexive defensiveness built into the Clinton political DNA. When faced with a tough story or what she views as an adversarial press, she tends to hit back hard — dismissing questions out of hand and, in the process, backing herself into a corner.

(That tendency was also on display last night. Asked why she hasn't given a news conference in more than 260 days, Clinton told Cooper that she was talking to him now — not a news conference — and offered this meaningless promise: "Stay tuned.")

For whatever reason, Clinton overcame that built-in defensiveness — at least for a night — in her conversation with Cooper about the email server. After 18 months of searching, she finally found the right answer on emails. Now she just needs to keep saying it.