Below is a link which should help you some: www.sparkpeople.com/resource/nutrition_art
Have you talked with your Dr about this? There are various reasons for this, medical and emotional, and so you really need a Dr's input, and a Dietitian to ensure that you get the vitamins and minerals that your body needs but won't be getting enough of. My suggestion would be that while you are having difficulty you get something like Ensure or Fortisip which is a 'meal replacer' drink that is often used for very sick people. I am not suggesting you use it instead of food, but as a drink. I THINK they are about 300 calories each.
If there is emotional stuff going on, then ask your Dr for a referral to a Therapist to help you. My own mother was having very similar problems to you but would sometimes go the whole day with no food. I couldn't get her to eat, even tho' I told her her life depended on it. Her comments would be I'm not hungry, so I don't need it. I KNEW that she had been down emotionally even tho' it wasn't obvious to others, and finally got a Dr to listen. (She hadn't been loosing weight so the Dr didn't see there was a problem.) She is now being treated for depression and her appetite has returned.