Maybe it’s an issue with glucose metabolism. Getting irritable, tired, weak etc shortly after meals sounds like hypoglycaemia to me, and would explain why you feel better after eating simple carbs. Wildly unstable blood sugar really isn’t good for you, and can actually make you more susceptible to infections. Women with vaginal thrush have been shown to have poor glucose metabolism.
Starving yourself is a disaster if this is the case. You need to eat balanced meals and stop yourself from going hungry to keep your blood sugar level. Lay off the junk food and eat balanced meals with a mix of protein, fat and carbs. Protein stimulates insulin just like sugar, and can lead to a crash when eaten in large amounts by itself. Caffeine stimulates metabolism and can lead to hypoglycaemia if taken on an empty stomach. Stress can cause dips in blood sugar too, which is one reason people binge on sweets for emotional “relief”. Sleeping at regular times can also help normalise hormones that can influence blood sugar, e.g. cortisol.