Can you please explain why bulimia is so hard to quit?

Because of denial: Bulimia, as anorexia nervosa are caused by a variety of genetic, serotonin imbalances and brain hormones (brain derived neurotrophic factor) which cause the patinet to have a self distorted image. The do not recognize nor are they able to admit they have a problem. Intense psycotherapy and behavioral therapy are needed to control the problem. Full cures are rare, but controil is possible.