An obsession about healthy eating is an obsession like any other obsession. Besides what you consider "healthy eating" may well be poisonous, because it is very difficult to work out what is healthy to an individual biochemical person.
An obsession of whatever form is usually activated by the release of excess adrenaline - the worry hormone. Adrenaline is also a "focusing" hormone - it forces you to focus on the "enemy" in case of an emergency.
But unfortunately there is no external enemy in anxiety attacks and we need to analyze why the body is producing too much adrenaline at the wrong moment. Please read:
Obsessive Compulsive Disorder (OCD) and Nutrition.