I feel that any positive feelings I have about her have shut down. She has indicated in her entries that she is depressed and is thinking of death. Would counseling be an option here, and, if so, should it be joint or just individual? I don't think that she would go along with it. Do you have any ideas on how I should approach her about this?
I think your top priority is your daughter's depressed state and thoughts of death. It is not uncommon for teens to be depressed about many aspects of their rollercoaster life nor is it unusual for them to think of death as a way out of their despair. Just because these are common behaviors that doesn't mean they are any less troubling. I would make an appointment for yourself with a therapist as soon as possible. Make sure this therapist has a background in dealing with family situations like the one you are confronting. I don't know enough about your daughter's personality or the current status of your relationship with her to offer the kind of advice that would be personalized. You can't sit with this knowledge and worry about her taking her life - you and I know that. I always take kids' talk of death and depression seriously and if you have noticed other recent behaviors in her that have indicated a more distant, troubled state I would give her diary writings more regard.
My hope is that something good can come out of your indiscretion. She will be furious that you read her diary. You can get past that and begin to focus on repairing your relationship with professional help. Please set up your private appointment soon and go from there. You cannot linger on your personal hurt based on what she wrote in her diary. You need to take responsible action for the safety and well being of your daughter. The complex nature of what brought you and her to this present point in your relationship deserves to be worked on - for both of your sakes. Give me an update if you have the time.