I tried my best and still no improvement!

I (26 M) and my GF (26 F) are high school sweethearts and have been in a relationship for over 9 years. There is no one in this world I love more than her. But during the past 3 years since COVID, she was diagnosed with hypothyroidism and depression, and she has been struggling with body weight issues since. I have supported her through all this for the past 3 years, but there hasn't been much improvement. Her weight has increased even further. I still love her as much as I did before, but I feel that I am not physically attracted to her as before. I am not trying to be fatphobic or anything, but I don't find thick women attractive. We still get physically intimate because I don't want to make her more insecure by telling her I am not attracted to her. She keeps asking me whether I still find her attractive, and I say yes. I feel terrible for lying to her, but I think I would feel even more terrible if I told her the truth and it hurt her, making her mental state worse. Other than the looks or physical attraction, the effect this is having on her health is really concerning to me. I keep thinking about our future, and I feel like it is going to be very bad health-wise. From time to time, I talk to her about reducing weight, going to the gym, and taking care of herself. But the depression and mood swings make everything more complicated. What should I do? I have been supporting her through these tough times for the past 3 years, and I am willing to go even further if necessary. But when I don't see any results or even small improvements after all this time, it starts to affect my own mental health now. If anyone has gone through something similar, please give me some suggestions. I am sure that this is the woman I want to marry. She is my best friend and the best human I know. I am really worried for her