AD's will make you gain weight, and retain it. AD weight is a pain in the ass to lose. But it shouldn't be this hard, should it? I gained on AD's but I was still able to lose and stay a healthy weight with them once I got used to my new body chemistry.
Have you talked to your doctor about a thyroid problem? At least get it tested to see?
Also I read about the calories. I noticed I lost far less when I was counting calories and going to sleep hungry because I hit my max calories for the day.
This worked for me, but it may not work for you. Here's what I did:
Throw out the scale. Stop counting calories. Stress makes you retain and gain weight, if you're stressing about the numbers and calories you're being counter productive, and not all bodies are created equal. Some need 1800 calories, some need 1200. Go by measurements, and how you looks and feel not arbitrary numbers. You're doing a life style change, not a diet. Eat healthy snacks, but eat only when you are hungry. When you don't feel the hunger feeling stop eating, don't eat an entire thing if you're not hungry for it. Also eat slower and snack throughout the day so you're eating mini-meals, not big meals where you might binge if you've been waiting for so long. Allow yourself treats, but in moderation. If you want chocolate, have some but instead of like a piece of cake, get a bag of fun sized bar and have a little mini-candy instead. That way you don't cave to cravings and eat a huge fatty thing. Eventually enough of eating healthy and craving for junk food will go away. And... my biggest vice, don't drink your calories. You can't out exercise a bad diet.
Don't expect success right away either. The body will variate weight depending on water weight and a variety of other factors. The first few weeks I started working out I actually gained water weight which was discouraging. But then I started losing. A healthy weight loss is no more than 2lbs a week, which is very slow. You didn't gain a ton over a short period of time (and if you did..doctor, now.), so you won't lose it like that either.
Anyway, dietitian is good start! :) Good luck.