I have trouble eating enough, too. I just don't enjoy it and never have. I don't want anything to do with food. A few things have helped me... I don't know if they'll help you, but I'll tell you what's worked for me.
1. I see a nutritionist once a month. She helps me to make sure I'm getting the right foods at the right times and in the right quantities. I know a lot about nutrition, but it still helps a lot to have someone to bounce ideas off of.
2. Smoothies! They can be made with low-fat, low-sugar ice cream if you just want the good nutrients, or if you need the calories, they can be power-packed with the best ice cream. I make them with fruit or with peanut butter and protein powder. I can get all the vitamins and stuff that I need in almost nothing but smoothies. Another good one is Ensure or Boost. Personally, I hate to eat, but I love to drink.
3. Feeding therapy. I'm doing feeding therapy at a pediatric OT clinic, and they're helping me adjust to new foods. Namely, I gag on fruits and vegetables. Since everybody needs them to be healthy, and I can't eat them, they're helping me with that. It's not exactly fun therapy, but I've added oranges adn blueberries to my diet, and I've successfully swallows carrots, honeydew, green peppers, lettuce, and celery with peanut butter. Definitely onto something!