If you have a restrictive eating disorder then losing any weight is not for you right now. You have to conquer the eating disorder and your thinking you are "fat". If other, independent people do not think you should do this, your husband and therapist who only want what is best for you (which you cannot be relied on to "want" because of your disorder), then I would go with them.

As krazibean says, being fatter/thinner has nothing to with happiness. Losing weight should not be a "goal" unless you are obese and facing medical problems as a result. Weight is not about beauty, it's about nourishing your body properly and being healthy.