Anyone who craves cornstarch or any other non-food items is said to have a condition called pica. Pica is sometimes an indication of a mineral deficiency or imbalance and sometimes anemia. Cornstarch is a corn derivative and is usually used as a thickening agent. It is non-toxic but can cause mineral deficiencies. Eating a large amount of cornstarch can paradoxically lead to anemia, ulcers, acid reflux, and mineral loss. To much cornstarch ingested can also lead the person to have severe constipation. If you have this problem, talk with a doctor as soon as possible. and get a thorough check up and blood test. Good luck to you.
This is pica. It could be caused by a mineral/vitamin deficiency. Typically, the deficiency is in iron, but it could also be zinc or vitamin B. You need to see a doctor and have him assess you for nutritional deficiencies. In addition, cornstarch can raise glucose levels and precipitate type 2 diabetes, so you really need this checked out and addressed.
Often, correcting the deficiency takes care of the problem. If it doesn't, you need to work on this as a type of OCD. Exposure and response prevention therapy is probably the most effective route for addressing OCD.