You can eat healthy on shabbos. i don’t know who cooks in your house, but prepare healthy food, buy the smallest amount of challah possible and tell the other person to remove it once you’ve had a bite. You can eat salmon, salad, matbucha, hummus, grilled chicken, grilled meat is fine too, don’t use unhealthy or sweet sauces and dressings, fill up on quinoa. No temptation = no fail.
I eat rather minimally and healthy during the week, and then on shabbos I can be totally undone, half a challah, kugels, cookies, cake. Comeplete abstinence is what works best for me, but if you find you must indulge, indulge Shabbos day and ONLY at the meal and wait two hours before taking a nap. Good luck.