How should you go about giving up an eating disorder assuming no help from medical organisations help groups etc?

You could buy self help type books. Some I recommend are "Eating in the Light of the Moon", "Life Without Ed", "Intuitive Eating", "The Food and Feelings Workbook".

You could also look up things on the internet to find what you should weigh and about how much to eat. On average, we are supposed to get about 2000 calories per day. If you are underweight you will need much more than that in order to gain weight. When I was in treatment, I needed 4000+ to gain 2-3 pounds per week. It was hard to believe how much/how hard it really is to put on weight. It's because your body is trying to balance things out and keeps getting used to having a higher amount of energy from food. If you are anorexic, it helps to have lots of small meals throughout the day (5-8) and to get in extra through drinks like boost or ensure plus. If you are bulimic or coe/bed, it helps to get in about 3-5 avg sized meals/snacks. Sometimes it helps to make a plan for yourself of when to eat. For others it helps to try to focus on eating when they are truly hungry and learning how to listen to their body again.

Finding other things I was passionate about / other things to focus on helped me too. Journaling, dancing, painting, or other ways of expressing yourself helps as well.

There are some great online fourms like the one on somethingfishy.org that offer lots of online support. Do not go to fourms that look like they are "pro-ana" or encouraging eating disorders. I promise they will only make things worse.

It's hard to do without help from other people though. It really helps to have someone there to help you with figuring out why you have an eating disorder and figuring out ways to get your needs mets (& other coping mechanisms) without destroying your body. Dietitians and doctors help a lot too with the physical aspects of it and what/how much to eat, too. I know it can be embarrassing to see someone for this, but keep in mind that they have seen many people before with the same problem. I have also noticed that a lot of professionals that specialize in this once had eating disorders themselves and have an idea of what you're going through. Good luck. =)