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.