I was officially diagnosed with hypoglycemia about 22 years ago (but I'm sure my body had problems handling sugar probably since puberty).
Before suspecting I could have a problem with my blood sugar I suffered a great deal due to depression, anxiety and more. I thank God for putting in my hands books that pointed me in the right direction as I believe discovering I was hypoglycemic probably saved my sanity and maybe even my life. I am now 54 and still deal with depression and anxiety (though not as bad as in the past and if so only for a day or two at a time), in part because it has become very hard for me to stay on the diet, stress makes me crave sweets and starches like crazy and there hasn't been a shortage of stress in my life ever since I had children. I suspect I had "adrenal fatigue" so my body reacted very poorly to the hormonal changes of menopause. Since I've been interested in nutrition most of my life I've tried many different supplements which have helped to a degree but I really haven't found my "magic key" yet.
So, naturally, my suggestion to you is that you buy your daughter a good book about hypoglycemia as perhaps when she sees the connection between her diet and moods she'll feel motivated to improve her diet. I would recommend that you first go to a public library and check out at least a couple of books on the subject to read yourself first, that way it'll be easier for you to choose one that you think will be easier for her to relate to.