I would definitely tell her how it's making you feel. And that it's not helpful. I assume there are other topics you'd want to spend time talking about instead? If so, it might help to say, "I want to be talking more about x and y, but when you spend so much time on weight, I can't talk about them."

My T has also learned (in part due to how I reacted and in part from telling him) that if he keeps bringing something up, like my needing to exercise more, it's not helpful to me and can backfire (it can make me withdraw from him, for example). He will occasionally throw in a brief statement like, "You know more exercise could help you sleep better. And that's all I'm going to say about it."

So maybe it could help to say that you know all these things, that her continuing to repeat them doesn't help. And could make you reluctant to bring up some topics/feelings, like saying you have no energy or are feeling ill, because then you're concerned she'll just go to the eating thing again.