Depression can have a genetic link. Your daughter may feel depressed due to other things such as family dynamics, school stress, diet, inflammation, etc.
It may be helpful to talk to her and find out what her symptoms are, what she thinks about, what she attributes her mood to and discuss what she may be willing to do to feel better. Depression can be even triggered by hormones if her estrogen is low. Most importantly, don't blame yourself for her mood because that could hinder your ability to talk to her about it objectively.
Depression is a clinical condition and you don't really cause it in another. Her reaction to you or events in her life can be affected by her coping skills, her self esteem, wants/needs that may not be met, etc.
Not sure if under things tried you only listed those things you've done or something she's doing as well. Counseling can be an option for her as well as family counseling if you feel/believe that there are some unresolved issues b/w the two of you that can be affecting her mood.
It is also helpful to educate her on what the condition/disorder is. She may not feel comfortable always talking to you and needs to have some sort of an outlet to do so - to work her issues through. There are some free online support groups.
When you speak with her, you can start by asking her how she feels on a day to day basis, does she have days where she feels worse than others and did something happened that day. Does she wake up feeling down, does she have crying spells or thoughts of wanting to harm herself, is her mood worse in the fall and winter, etc. She is old enough where she can elaborate and speak to you about her experiences and let you know what she feels comfortable with trying i.e. medication, speaking to someone or both.
Depression - Teen Support Group - DailyStrength
It may be helpful at some point to have family counseling (working together) on these issues you've listed. Meanwhile, try to keep your communication open with her. Let her know that you don't have all the answers she may be seeking at this time but would help her along the way as much as you can. Depression can exist within a family and the two of you are already taking steps to manage the condition and improve your mother-daughter relationship.
You could speak to her about what you find helpful to deal with your mood i.e. art, music, exercise, staying socially connected, journaling, meditation, etc. In a way, you can still be a positive role model to her depsite your depression. She would have to learn to separate the individual from their condition.