For context, my girlfriend and I have been dating for about a year now, and we're really close, especially when it comes to talking things over. We would both describe ourselves as nerds, and would never normally really care too much about how other people judge us.
Fairly soon after we'd started dating, she revealed to me that she often didn't feel brilliant about her body image. I responded by repeatedly telling her how gorgeous I find her, which, after a bit of time, appeared to clear things up pretty well. However, this has recently reared its head again, and I would really like a way of finding a cure for this, rather than a bandage for it.
Even without being totally biased as her boyfriend, she is nowhere near overweight (she's slim, and is a regular, if casual, runner), but I can't seem to get her to believe that or understand it. Things did develop a little bit recently, when she, whilst we were discussing this, having thought about it for a bit, thought that it might come down to her being worried about not being healthy enough - which was why my attempts at reassuring her as to how beautiful she is has reduce in its efficacy.
The other factor in this is that family members have, in the past, not helped her in her body image (or just in general about her appearance - telling her that I won't like her if she doesn't put makeup on before dates with me - although I have convinced her that it's entirely the opposite, I'd be more offended if she did!) - for instance in describing her, in relation to her even thinner sister, as 'the stocky one' in the family. One family member in particular has hypochondria, especially about obesity, which has led her to the opinion that almost all of the country is obese.
Basically, how can I make my girlfriend feel better about herself? I am slightly worried that, because she can be extremely determined, this might end up reaching the point of anorexia if she puts her mind to it, or, that if I convince her not to do anything, that she might end up feeling worse about herself.