P.,
I'm so sorry for you and your daughter. Another poster is right - you wouldn't be here asking for advice, praying, etc if you were the "worst mother in the world".
Here is a website you may already be familiar with: http://www.suicide.org/support-groups/north-carolina-suic...
Although the "survivors of suicide" groups are often for family members whose children have succeeded, if I were you, I'd give a call to one of the groups (another W-S group here: http://wakeforestsos.webs.com/) just to talk to them. I know there are plenty of other parents dealing with suicidal children (I have a friend who's dealing with it right now, for instance), and it would probably be very helpful for you to talk to them.
Having a suicidal teen at home has got to be one of the most stressful things - you feel as if you are on red alert 24/7. But there is no way you can be with her and protect every moment of the day - it wouldn't be good for her even if you could. Please do continue to seek help, both for her, and for yourself. My friends who know me as anti-meds would laugh if they heard me tell you that if things don't get drastically better very soon, please consider taking her to an actual psychiatrist (not pyschologist ). Even if you just go to gather info, and to rule things out. Imho, best to have covered all the bases.
I've suffered from clinical depression for most of my life. From that perspective, I can tell you that I firmly believed that the world (incl. loved ones) would be better off without me. No amount of rationalization helped, because that's just it - suicidality is IRrational. What I mean is, even if you were Mary & Mother Teresa combined, - even that perfect parenting couldn't necessarily keep your daughter out of this situation.
On the other hand, had I had someone advocating for me when I was young and telling me they loved me and would try to help even if they didn't know how, it would have helped a lot.
Just as living with teenagers can be exhausting, so that exhaustion can increase when you're caring for one with chronic illness (in this case, I'm assuming that your daughter is clinically depressed - pardon me if I'm wrong). Be as gentle with yourself as you are with her, and love yourself as much, too. You definitely are not alone, and by talking about it, you help to take some of the shame out of suicide and depression. Thank you for that.
Blessings and hugs........