I think at this point I should clarify a little further.
Yes, originally this post was talking about my attraction to my girlfriend waning. I understand that that is on me, that is my psyche and my preferences in a partner. That is not my fault, and unfortunately not something I can change.
However, the initial ask of this post was to try and determine ways of fostering a healthier lifestyle. Because from everything I've read by couples counselors and relationship therapists and, you know, actual experts in this field, that is how a healthy relationship grows. Losing attraction for your partner is not grounds for ending a relationship. Doing nothing about it, however, is, but not from me. I want to be in this relationship, because she is my favourite person, so I want to feel attracted to her again. Love and attraction are not mutually exclusive. Until a lot of you learn that, you will never have a meaningful relationship that lasts. And, for what it's worth, I don't find stick thin people attractive either.
What's more, as I've already mentioned, she knows she's let herself go. She knows she's living an unhealthy lifestyle. I know this because she has told me herself, without any prompting from me. She knows this is detrimental to her physical health, her self-esteem, and her mental health. I didn't think it prudent to include this in my original post, but thanks for assuming this is all down to me and my surface view of her and its got nothing to do with the way she feels about and views herself. Some of you are obviously happy being overweight. That's great, more power to you, and I hope you stay happy, and you find/have someone who finds you attractive the way you are and I hope you never want to lose weight one day otherwise that someone might not find you attractive anymore and by your logic they should leave you for that. My girlfriend, however, is not happy being overweight.
Hence my ask. She wants to live healthier; I want her to live healthier. She wants to be happy in herself; I want her to feel happy in herself. It just so happens that being a healthy weight is more attractive to me than being overweight.
A big thank you to all those of you who have actually been supportive and given me some actually constructive comments and sound advice other than "lol leave her". I have taken those comments on board and will try and do what I can to further foster a healthier lifestyle for me and my girlfriend.