That is one of life's hardest things to deal with...and one of the most painful. My heart goes out to you...because that is one of the things in life that only time can help. People who usually contemplate doing something like that are suffering from severe clinical depression...and if she had a predisposition for depression...then it is something she would have dealt with whether she knew you or not. So dont blame yourself. She may have had those kinds of thoughts long before she met you. And I know that ...knowing that wont make the situation any less painful...and it wont hurt any less...there is nothing that anyone else can say or do to make the death of a loved one hurt any less...and grief is a natural process that takes time...and eventhough you may never forget her...with time your heart will start to heal and you will be able to try to move past all the terrible pain...and think only good thoughts of her...remembering all of the fond memories.
I am so sorry to hear about your painful situation...and I will keep you in my prayers. God bless and take care.