need to vent and maybe gather some advise...

I love my daughter.
Any mother knows the love I have for her, because they feel that love for their own children.
Everyday I thank my Heavenly Father for her, and for entrusting me to care for her, to love her, to teach her.
She brings both Lance and I so much joy.
Joy that can't be replaced with any one item or material thing.
She spent her first nights away from us the weekend of our anniversary last month, and it almost killed both of us.
She's the reason for me waking up, making breakfast, getting ready for the day, taking care of our home.
Everything.
But lately,
She's been a terror!
Please, someone, tell me this is a phase.
She throws tantrums, she's bit me a couple times, and in return I bit her and left a mark.
i felt this small and like a horrible mother
She points her finger at me and tells me "No, no mama," when I tell her to do something, or to not do something.
She's ALWAYS running away from me.
She corner's other kids and makes them cry.
Then, I want to cry because every other mother is thinking my child is mean.
But then she'll say, "I sawry Mama," and look at me with her blueish-grayish eyes.
She'll hug my neck so tight, I don't want her to let go.
She'll cuddle me when I don't feel well.
She'll give me the best most juciest kisses ever!
And the first thing she says in the morning, in her little soft voice is..
"Mama, mama, mama..." until I go and get her out of her crib.
And once again, she hugs me, wants me to hold her, kisses me with that dragon breath of hers, and I don't even care cause she's my baby.

Someone please tell me how to handle her little attitude!
I hate spanking her.
She's only 21 months old.
Does she fully understand why she's getting spanked?
Or does she think:
"When I get upset, I can hit too."
She doesn't understand time out.
Although, she hates when I make her sit in her crib.
So maybe I'm doing something by doing that.

Any advise?
This is a phase right?
She'll grow out of it and be my little angel again until her teens, right?

{sigh}

Motherhood...
it's a wonderful thing.