I gain weight eating less than 800 a day, and NOT in starvation mode. perspective?

I have an under-active thyroid, and ever since I got severely sick at age 13 and literally did not eat for a month (then took 6 months to resume, then vomited daily for the next 3.5 years. doctors never found anything. a few said I was crazy, actually.)

Ever since then I don't eat a lot.. my personal level is around 800. It is very rare since the age of 13 that I have eaten over 1000 calories a day (i am 23 now). Most of it is vegetables, fresh fruit, salad with natural/low cal dressing or balsamic vinegar, some lean meats, plus s different vitamins to cover anything I might need. Forcing 1500 calories into my body a day for 6 months resulted in 25+lbs of weight gain (while working out 6 days a week). It took periods of juice and water fasting to bring my weight down from 150-175 to a healthy 120. Once I was there, for a period of a year, I ate normally (my normal), felt good, maintained a healthy weight of about 123 (I'm 5'6"). Then, suddenly, one week I gained 5 pounds. he next week 3. The next week 4. It continued on this way until I reached 165 pounds. It peaked about 6 months ago, and eating 800cals (usually less, lots of water. this isn't starvation mode for my body, even though many a doctor has tried to convince me of this, without ever hearing my history. they just assume everyone is the same i guess.) doesn't have any effect.

So.. I work out about 5 days a week, an hour+ of cardio, an hour strength training, a half hour yoga. My typical diet: Breakfast: OJ (a pint and a half), a medium or large banana, whole grain toast (1 slice) (don't always have this). I snack on grapes or fruit in the late morning, and have a cup of soup and maybe a small (palm sized) salad for lunch. Usually vegetables or low-cal fruit snack somewhere in here. For dinner it is usually lean meat (small portion) with a starch (small portion) and loads of steamed vegetables. I drink tea with stevia, and about 4 bottles worth of water a day, filtered. I take several supplements to cover deficiencies, including b-complex, b12, biotin, calcium, iron, D, and a heavy multivitamin. Also fish oil and sea kelp (iodine for the thyroid).

I am also pre-med with a concentration in nutrition so I am not naive in this area either.

So.. based on all of this.. why can't I lose weight without straight-out fasting on water or juice?