Struggling to start a water fast / how much can I lose?

Hi! I'm a 20 year old female with a bmi of 24.9. At my goal weight (which was my high school weight) I'd be at 19.4. I've gone on Water Fasts before, but I always messed up afterward by eating too much and regaining the weight. I won't make the same mistakes again. This time I want to go for 28 days, but I always seem to *need food* by 37 hours.. Does anyone have any advice for just waiting it out? How to find the discipline / emergency back up plans / etc?

Also... I'm trying to lose 35 lbs.. Would 28 days be enough? too long / too short? I'm not quite overweight so I'm not sure how much weight loss to expect..