Why are daughters mean to their mothers?

She's not mean anymore because she knows she can come to me whenever she is angry, sad, nervous, pissed off, anxious, scared, disappointed, bored, confident, in pain, hungry, lonely, excited, heartbroken, in love. So, she comes to me all the time. We talk, we argue, we cry, we laugh, we hug.. there's no room for her to be mean.

Psychology sustains that, generally, children have a competitive relationship with the parent of the same sex while they get along better with the opposite-sex parent(Oedipus complex). Maybe this could explain why a mother-daughter’s relationship can seem tense, especially viewed from the daughter's part.

At a very young age all children can be mean, harsh and cruel when their demands are not met. When, at the age of 5, my daughter asked for a sibling and told her that I could no longer provide her with one, she shouted at me that if I had taken better care of her elder sister she wouldn't have died. This stubbed me, but even at that point I felt her own pain and the need to blame someone. So, maybe, daughters are mean to their mothers when they feel pain, anger, fear…

Ive been pretending to be okay and acting as normal as possible, but Im actually completely heartbroken after a recent breakup. Its painful and really affecting me, to the point where I cant concentrate at work, Ive lost my appetite, I cant sleep, and It feels as if my whole world has been turned upside down. I loved him so much. He said so many cruel things to me and it made me realize he must not have loved me the way I loved him, or he wouldnt have said such horrible things. How do I handle the heartbreak and why cant I accept that he didnt love me and just forget about him?

From personal experience I would say that there's no straight answer to this question or not only one answer, it depends on mother's and daughter's personality, behaviour and age.

Thankfully, this was the only time she was mean to me. She's a teenager now. Of course, at times, she can be a little rude and disrespectful to me but, to be honest, I'd be worried if she weren't, adolescence is a tough phase for children.