Big hug, OP.
*Standard disclaimer that this is general advice, and my personal opinion, and for proper advice, go see your GP
Food/eating can be one of the biggest causes of tension between parents and kids.
I definitely think you are best off going and having a chat with a good psychologist who is knowledgable on eating issues (not all are, you might have to have a hunt around). I think they would be better placed to help you than a dietician. No offense at all intended to dieticians, but based on what you have described, it sounds like this is primarily a behavioural issue rather than a nutritional one.
Food refusal is often linked to anxiety. There is good physiological evidence as to why this is. Some of the same hormone pathways that are involved in anxiety are involved in digestion. Do you know what it feels like when you are really nervous, and your tummy gets all fluttery? And some people find that being anxiety can give them diarrhea (where the phrase, "scared the sh*t out of me" comes from. IBS (irritable bowel) is also strongly linked to anxiety. So, if your daughter has a tendency to be quite an anxious child, this might be impacting on her appetite.
Also, for a child, food is one area of their life in which they can exert control. Ultimately, they decide what they eat, or what they don't. If your child is feeling quite out of control in other areas of their life, sometimes this need for control can start to impact on their eating.
A good psychologist will have LOTS and LOTS of helpful information they can take you through. Depending on what state you are in, there are also eating disorder clinics which have a wealth of resources for tackling these issues (and may be able to provide some subsidised therapy). NOTE: I am NOT saying your DD has an eating disorder, or that she will develop one. That is not at all what I am implying, I am just suggesting them as a source of well-trained, knowledgable people who can point you in the right direction and provide support for you as a parent.
I think there are some negative habits that your daughter is forming, especially hiding food and sneaking/stealing food (eg bread), and these would be worth addressing too, as they do have some potential to escalate.
I just wanted to congratulate you on being so proactive, and caring for your DD. Eating issues can be very difficult to tackle, as a lot of parents think it is something their kids will grow out of, so put off seeking any advice or assistance. The best thing you can do is to act now and nip this in the bud.