“I am done with being a candidate.”
“I am not done with politics because I literally believe that our country’s future is at stake.”
I was a Clinton supporter in 2016, but I think her decision is the right one. She can still be a valuable voice in the Democratic Party, but it would be a mistake for her to run again.
Now we need to hear the same thing from Bernie Sanders. It was bad enough to watch him divide the party in 2016 (while claiming he was not a Democrat), and we certainly don't need him doing the same thing in 2020 -- or trying to badmouth Democrats running because they don't agree with him on 100% of the issues. He has made it clear that he doesn't want to be a Democrat and doesn't respect the Democratic Party "big tent" (which includes liberals, moderates, and conservatives). The Democrats need to be unified in 2020, and that will be much easier with a new candidate (without the baggage of either Clinton or Sanders).
It's not like the Democrats don't have a lot of smart and viable candidates -- they do. Just off the top of my head, I can name quite a few I could support (and think most Americans would find acceptable) -- Joe Biden, Caroline Kennedy, Elizabeth Warren, Kirsten Gillibrand, Tammy Duckworth, Al Franken, Eric Garcetti, Andrew Cuomo, and that's just a few of the possibilities the Democrats have.
We must defeat Donald Trump (or his replacement) in 2020 -- and that will be much easier to do if neither Clinton nor Sanders joins the fray.