If you've gotten checked out by your physician and you're healthy, then you probably need to look to your emotional/psychological self. That's where you'll no doubt find important reasons for your eating and weight problems.
Here's the twist: Going on a diet is a good tool for dealing with your deeper emotional issues. If you keep eating over them, you hide them. If you go on a diet and bring your eating to a better place, the emotional issues will have a chance to come out of hiding.
So, you can think of it like that. Instead of fixing the non-food issues so you can diet successfully, you can diet as a way to acknowledge and successfully fix the non-food issues.