Hello, mysoul! You've already received wonderful advice here. I'll just tack on my two cents.
Your daughter is hurting herself. She's rejecting herself by neglecting her body. All of us who've struggled with weight understand what a painful cycle it is to feel horrible about the way we look (or ourselves in general), and then we eat to comfort ourselves, which in turn compounds the problem.
I think the one thing your daughter needs more than anything else is probably unconditional love and adoration. Now, I'm not saying she needs to be enabled in killing herself; just that she should feel your focus in on her person and how much you love her, and not on how she looks. My guess would be that she's aware of her weight every waking hour as it is. ???
If you feel you need to have one more sit-down talk with her to make clear how concerned you are about her health and that you fear losing her, then I would definitely do so. Just be as adoring of her person as you can be in this conversation so she can walk away with the knowledge that she has your unconditional love, that you see her heart when you look at her, and that you just don't want to lose her early.
After that, I'd just be the best example of health you can be for her, without acting like dieting and looks are everything. And she shouldn't get the impression or the feeling that you're silently judging her, trying to make her to be like you. Just set a wonderful example for her while you love her for who she is.
I don't believe that we who have relationships with our parents, ever stop looking to them to be the example, even silently. If they're not leading well, we'll look elsewhere for our examples of how to live. If you and your daughter maintain a loving, accepting relationship, and she sees you being successful in your dieting efforts without judging her or pushing those ideas onto her, she'll have a good example to follow right at her fingertips.
Also, if you're eating good, healthy, filling foods, she'll see that losing weight is possible, doable, and when she's ready, she'll likely feel less intimidated to get started. She may even turn to you for encouragement.
I have to ask, do you think there's even a remote possibility that she's unaware of the health risks of being so overweight? And, do you think she truly feels good about how she looks, deep down, in her alone moments? Do you think there's even a slight possibility that she thinks you aren't at all concerned about her health? What I'm leading up to is, there's a great chance she already 'knows' but, as others have said, she's not chosen to make a change yet.
Pray for her! And love her with all of your heart. Be her example without passing judgement.
I'll say a prayer for you guys right now.
Love,
Alyssa