The first commentator summed up beautifully everything I wanted to say. Stay out of her life. I get it. You are her mother but what you are not understanding (though I'm sure your daughter has been telling you for years) is that you are her problem. Your letter encapsulated everything I hate about my mom. Ultimately, I know that not having my mom in my life continuing to shame me and make me feel like garbage over my weight is the best for me but my love and this pull I have to her continues to force me to gravitate to her negativity. And it hurts. Every single time she does it. And I keep coming back for more. No matter what else I have accomplished in my life - my weight creeps into the conversation. And at the end of the day no matter what your daughter achieves she is not having relationships because you have ingrained her to believe she isn't worth love if she doesn't look a certain way. She has a low self esteem and anxiety because at the root every time she wanted to do something she's told fat girls don't do that or fat girls don't wear that or are you sure you don't want to cover that up? Years and years of this eventually cripple your ability to go out and take control of life. And it feeds a false narrative that if we were suddenly thin life would be great. The truth is the damage is done.
Pay attention over the course of a week or two. If every conversation you have with your daughter eventually somehow lead to weight related conversation then what is the basis of the relationship you have with your daughter? Connect to her on some other level. Do something else. Try to heal that relationship instead of trying to fix a problem you think she has. Let her make her own decisions.