Wanting to give up

I'm new to this board, but I'm hoping I will find some help.

All my life I was an athlete weighing around 145-160(5'7) pounds, but having a lot of muscle.Then I went to college. I didn't work out at all my freshmen year and my weight went up to 170, but I had no muscle and the difference in size was very obvious. I lost a lot of weight in the summer of '04 and went down to 140 pounds(at a very healthy rate of 2 lbs per week). My ED started August of '04. I started trying to lose more weight. I ran 10 miles (at least) every day 7 days a week eating 1000 calories or less. I was always tired and cold and at some point decided to do something about it. At some point in October I started eating more(1500 calories), and lost a lot of weight (went down to 120). Then I started training for a marathon and my weight went up to 124-26 pounds. I can't remember a time when I was so happy, not just because of the weight, but I felt good about myself and my weight and everything seemed to be going well (This lasted until march).
After the marathon my weight "jumped" to 130 pounds and I freaked out and cut back on calories (1000) and still ran 10 miles a day (6 X per week). I had not been able to lose more than 3 pounds.
I am very unhappy and went to see a counselor for a different reason, and he said I had to see a nutrionist. She has me eating 1800 calories a day and taking birth control (because I haven't had my period in 13 months). My weight is going up, every day I gain more. I am up to 133 pounds! I am freaking out. Half of me knows I need to keep doing this, but the other half wants to give up and stay away from food. Now, if I eat the amount the nutrionist wants I am unhappy because of the weight gain, and if I don't I'm unhappy because i'm not doing what I should. I am very frustrated and just want to get back down to 125 pounds.

Why am I gaining so much weight everyday? The nutritionist even said I should be losing not gaining? I just want to give up.

Has anyone else done this?