I am no expert, but I would increase your calories. You should never be below 1200 calories a day from what I have read. Also I would do away with the water pill and let your body naturally balance itself out.
I have more weight to lose than you do, but staying right at my calorie allowance (I have my settings for a 1 lb loss a week which is what you need to have yours set on based on the little bit you need to lose) and sometimes I eat back part of my exercise calories! I even have bad days where I even go over the 500 a day deficit. Even with all of this, I lose at least a lb a week!
In short, do away with the water pill and make sure your calories are at least at the 1200 calorie point. You should be eating your allowance for calories though not shorting them too much (probably not any with the amount of exercise you do), and make sure your settings are at 1 lb per week so the calorie range is correct.
It sounds like your food choices and exercising are good though.
This is just a thought. Maybe try it for a couple of weeks and see if it works better for the weight loss!

Also you will have better results if you mix up your exercise. One day on the treadmill slow and steady, the next do intervals, the next go to a yoga class or do a DVD. Your body is an efficient machine and will adapt (conseve energy) if you are doing the same old same old every day.

Contrary to the prevalent thinking you do NOT need to slog an hour a day on a treadmill to get in shape. It's probably actually holding you back at this point.

How long has this been going on? I know this can be frustrating - it's hard work, especially when you're hungry all the time like you probably are.

I'd eat your minimum suggested calories + any calories you burn off and focus on exercising. Once you build some muscle, your metabolism will rev up (muscle burns fat even while resting) so it will be easier to lose weight and keep it off long-term. Also, you'll look and feel better no matter what you weigh.

Did you take measurements maybe you are losing inches and gaining muscle.

Probably saying what's already been said - but you are putting your body in starvation mode when you eat so few calories so it's refusing to lose weight.
Honestly most people need to eat more than 1200 to lose weight (generally a woman can get away with 1400 - 1600 and still lose 1/2 - 1 lb a week).
I would try eating in that range for a few weeks and eating your exercise calories back and see what happens. You might initally see a gain because your body will want to hold onto all the calories after being deprived - that's why you should give it at least 2 weeks if not a month.