The muscle line going down her rump, her puny defined neck and shoulder, and pointed and stunted appearance tell me she is seriously underweight and malnourished. She is almost 2 and looks no older than a yearling. Id imagine you can probably see her ribs if she were shed out properly(lack of which is probably due to malnutrition). Horses happily eat hay constantly, even at young ages. So either there is something seriously wrong with her health wise or your hay is extremely poor quality.
If you don't know much about horses you need to find an experienced horseperson or vet to help you out with her nutrition.
Im not trying to be rude, but I am concerned for the animals wellbeing.