Flowers in December is essentially about people seeking nonexistent things out of a relationship. I usually don't take Mazzy songs line by line, but this is one of my favorite songs, so I'll try that here.

Before I let you down again

she's upset him before by not being or doing what he expects. This time she wants him to foresee that and also to cherish the "calm before the storm", so to speak.

I just want to see you in your right

She wants to be reminded of why she loves him and how they used to get along.

I wouldn't have taken everything out on you, I only thought you could understand

She wrongly assumed he had a capacity for sympathy towards whatever she was feeling.

They say every man goes blind in his heart

Part of love is losing all sense of direction.

They say everybody steals somebody's heart away

When you "steal somebody's heart away", the stealing means that the "victim" will lose their ability to love in the act of loving.

And I've got nothing more to say about it, nothing more than you or me.

"This is what's happened to both of us."

Send me a flower of your December

"Give me a sign you still love me, even if it's impossible."

Save me a dream of your candy wine

"Let's remember why we love each other. Let's think about all the goals and fantasies we had."

And I've got just one more thing I can't give you, just one more thing of mine.

Cryptic. What is this thing? Her heart or her life-size Tony Bennett standup? Animal vegetable or mineral? I'll just take these lyrics to mean he wants what isn't for sale-- even though he traded his heart for it.

...and I've been wondering why you let me down

She's clueless as to why he doesn't know the score that's in her own head. She thinks she's sent all the right signals,so what's wrong with him?!

And I've been taking it all for granted.

She never stopped to appreciate everything they used to have. His heart was simply a kind gesture, his understanding a given.Now they're both gone.