Hey, I don't think this has to do with eating healthily. If you say you are eating well, that might not even be the case. I know this one girl who has always been overweight and regularly sees the nutritionist. She just recently leveled off her genetically (!!!) high cholesterol to a normal level but that took her like 3 years. I know someone who had a great bod and flared up because of a rare and incurable thyroid problem. I know someone who was naturally skinny and underweight and forced by her parents to take weight-gain medication to release hormones, which messed up other hormones. Now, she doesn't have that body anymore.
For me, eating a little bit everyday makes me hungry, but I do that in the effort to limit my calories. Try to follow up that regime, at least not to allow yourself to gain more. Keep in mind that when you eat too little though, your body will consume muscle (which is made to remove excess weight) to take in energy. Then when you eat, your body will try to make you eat more to recompense for the low level of energy you hard before, therefore making you gain weight. And for me, whenever my mind tries to make me eat more, I try to focus on eating slowly and going against what my mind wants to say. (It works for me but I don't know about anyone else)
Here are some tricks:
1. Your stomach is approximately the size of your fist so eat that much food at each meal.
2. Don't eat at big chunks three times a day. (Breakfast, Lunch, Dinner) Break it up into little snacks throughout the day. Eat every 4-5 hours. (Unless you have diabetes, in which you MUST eat every 2 hours. Otherwise you'll die.

)
3. When you exercise, make sure you exercise the whole body, not only one area. Exercising the whole body will more efficiently remove excess. (I'm trying not to refer to it as fat. I don't want to hurt anyone's feelings XO ) Also make sure you sweat, since sweat will remove toxins and improve mood, weight, and overall health.
I think that in your case, it's either genetic, thyroidal, hormonal, or etc...
Try the doctor. Take in a visit and see what is truly the problem. There are people gifted with curvaceous figures. But there are also people gifted with a strong mind. Be one with a strong mind. If you can accept yourself for who you are, that's strong. If you want to get rid of your problem, focus your strength in it. If you believe in something and put your faith in it, you are going to win.
Good luck!!! It will be hard but if you go up against your problems and fears and beat them down, then you can truly be proud of yourself!