Your father is at fault, here, not you. From your description it doesn't sound like his girlfriend is very healthy at all. If he finds that attractive, fine, but he has no right to try and force his ideals of unrealistic 'beauty' onto his daughter, especially if it's bad for her health - physical or mental.
I know how you feel, really. I have a lot of troubles with my weight and self-image, although I'm lucky enough to have a boyfriend who encourages me and says I'm beautiful, it still bothers me. My dad's girlfriend is a lot thinner than me, too, so I used to avoid eating around her...
Please try not to consider resorting back to your old habits. I find that if I do something like an hour or two hours of karate a week, I can feel healither just by knowing I'm doing it. If you perhaps pick up any small activity, like going for a half hour walk in the evening on some days, regardless of whether you need to or not, you can remind yourself that you're being healthy, exercising, and there's nothing wrong with that.
Tell your father that you're healthy and he should be encouraging his daughter to look how she wants, and feel comfortable with herself. Failing that, I think I'd tell him to be quiet and judge himself by his new standards, first.
And please try to remember, a person's size doesn't change who they are. Whether you're fat or thin, you're still a good person, and although it doesn't seem like it, that's what the people who know you think.
If you really need to hear an honest, independent opinion, you can always try showing your picture to someone online who doesn't know it's you and asking them what they think.
Alternatively, work out your weight vs your height and use a BMI (body mass index) calculator to work out what your BMI number is. There's plenty of websites to do that on, and it will tell you whether you're in the healthy category, underweight or over. As long as you're healthy, you can tell yourself - and your Dad - that, and KNOW that you don't need to change.
