Eating habits

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I have absolutely horrible eating habits. When I do eat, I eat lean proteins, veggies, fruits, and yogurts, but I seem to be only getting up to about 500 calories each day. Unless I force myself to eat until my stomach hurts, I can't get over 1000 calories! I naturally dislike very sugary, greasy, or fattening foods, so I rarely overeat. I just can't seem to eat enough! Any tips?