I'm not sure what to do about a "screwed up system" (it sounds like your doctor was very unhelpful indeed), but I have read that if you eat at maintenance for a week or so then your system should reset itself. (This is barring any metabolic problems- have you had your thyroid tested, for example?)
I started out by calculating my daily calorie needs to maintain and lose weight. Put your details into this form: (I'm not endorsing the company, it's just an easy way of working out your numbers) Your BMR is the number of calories that your body needs to function if you do nothing except lie in bed all day (to do things like keep you breathing and your heart beating). Your maintenance calories, which incorporate your activity, are to maintain weight, and you can adjust to lose weight. It depends on your own personal goals, but I aim at a deficit of between 500 and 1000 calories a day, which in theory (these numbers are a strating point, and can be adjusted if you're not seeing the loss you want) should lead to 1-2lb lost a week. (The more exercise you do, the more you can eat, in essence) You shouldn't eat under 1200 calories a day except on medical advice, and a deficit of below 1000 isn't a good idea as you want to keep your metabolism as fast as possible. You shouldn't aim to lose more than a kilogram (2.2lb) a week, unless you're particularly big (loss should be 1-2% of total body weight) as too much loss can be unhealthy (from muscle rather than fat) and probably won't be sustainable.
The two most useful tools in calorie counting are a calorie counting program (e.g. myfitnesspal, fitday, or the one I use, cron-o-meter), and measuring tools (ideally digital scales- I have ones that weigh to the nearest gram or millilitre). The more precise you can be about what goes into your mouth, the more control you have over your weight loss.
As to what to eat, if I were you I'd start with that plan now that you've bought it, as you don't want to waste money. Other than that, I try to eat a healthy diet (lots of fruit and vegetables, good amounts of protein, a few serves of dairy), but basically eat whatever and whenever I like that fits into my calorie goal. I look on my calorie goal as a budget- that's what I get to eat/ "spend" in a day. If I had a lot of high calorie food, I'd go hungry (I have a very big appetite), so I eat a lot of low calorie density foods (for you, if you snack on your catering food and work the way I work, you'd take the amount of calories in what you've eaten off what you're allowed to eat for today- or think about it before and ask whether it's worth whatever percent of your day's food for what you're about to put in your mouth). If I meet my nutritional requirements, I get treats (chocolate, home made ice cream, etc).
I find to keep control of my diet I have to incorporate variety in my diet and treats, so I've come to get to know recipe websites very well (I look up low calorie recipes, adapt them, and use either my calorie calculator or an online recipe calorie calculator to work out how many calories I'm eating/ estimate how many calories I'll eat to see if the recipe's worthwhile making). I also make lower calorie treat food, and incorporate typical treat food in my diet in very small amounts. This doesn't work for everyone- there are people here who need to go completely cold turkey on treats (personally, that's a great way to get me to break my diet).
I'm all about "do what works for you"- I wouldn't recommend anyone follow my plan exactly (although you're welcome to look and steal ideas- I love it when people read my diary), and indeed calorie counting doesn't work for everyone (I think in theory it should, but it doesn't seem to). I strongly advocate being aware of these things, at a minimum, but then using that knowledge in a way that fits into your lifestyle. If you're not a big eater (I'm a
pig), then you may need to incorporate more high density calorie foods into your diet than me. You may not need to snack like I do, etc.
I also recommend getting into exercise- strictly speaking you don't
need it to lose weight, but it's a good idea for health and will help your body composition (ie, will help you maintain more muscle and therefore have a better shape at the end).
Hope this helps
.