Mercy or not??

How do you know when to show mercy or not?
My 15 yr old daughter is a very red, RED. (I am a very blue BLUE) She is difficult to handle and sometimes even she doesn't like the way she is.
She fights me all the time but sometimes I think she is fighting herself. She is quite angry about the state of our family and doesn't want to get back with her dad. She knows that we are going to live together again and this is upsetting her alot.

I'm not good at Parent Counselling Sessions yet, especially after I tried one with her and she got so angry .
She told me very clearly that she doesn't appreciate my blue emotions so now I am too scared to try again.

My 13yr old daughter is eager to be back with dad and tells us how excited she is. I think this could be making the 15yr old feel guilty?? (Maybe) Because she doesn't feel this way.

We are all very anxious right now and home is quite tense.
I don't know how to help the children when I don't feel like I am holding 'it' together very well myself.

Any ideas welcome and much appreciated.