She struggles with transitions

We are having a tough time with Catherine right now.  I feel like she is really struggling, and the tough part (and at time even frightening part), is that we aren't entirely sure exactly what is causing this.  I know there are some big transitions, and she has never dealt with transitions well.  Although she wasn't close with Andrew's brother, it is still difficult to lose a loved one.  There has been the transition of having Thomas in school with her which she doesn't love, and of course Robert has left.  I think she forgets how much stress and chaos he brought to our lives, and I also know that to a certain extent, it doesn't matter.  She adores her big brother and is sometimes blinded by the things that were tough.  On the other hand right now, her younger brother can do nothing right in her eyes and is the worst person on earth as far as she is concerned.  Andrew and I are struggling with her reactions to things.  Parenting teens is by far the toughest thing I've ever experience in life.