WARNING: Long post ahead. I do that, because I don't want to just prescribe stuff, I want you to understand the pros and cons of the decisions you make exercise and nutrition-wise so that you can make informed decisions of your own.
To be honest, after reading your post I'm still not entirely sure what it is you're doing training and diet-wise, but I'd be fairly confident that one of the following is going on:
1) You're doing lots of training but you're eating at maintenance calories, so not much is going to happen with your body composition.
2) You're eating at a calorie deficit so you're losing weight, but your training's not geared to effectively preserve muscle mass thus your body composition stays about the same as you lose weight - less weight, same bodyfat%, if you will.
3) You're training too much and eating too strictly, you're generally highly strung/put unrealistically high expectations on yourself/easily and constantly stressed, and it's all reaking havoc on your progress.
To break it down, to lose weight you need to be in a calorie deficit, ie consuming fewer calories than your body needs to maintain its current weight. You shouldn't try to go overboard on that deficit, as the further you go below your starting maintenance, the more sharply your hormones will respond to the situation, causing your maintenance level to drop. If, for arguments sake, you start out for 2,000kcal/day maintenance, and then go an an extreme 800kcal/day diet (which a lot of girls do), at face value you've just made a 1,200kcal/day deficit, which is 8,400kcal/week, or ~1kg/2lb weight loss per week.
However, your body will notice the sudden and extreme absence of food, and will respond by lowering your maintenance level of calories. Maybe your maintenance drops to 1,000kcal/day and progress happens very uncomfortably and very slowly. Maybe it drops all the way down to 800kcal/day, and absolutely no progress occurs whatsoever.
So you need to be sensible with your diet. If you're eating extremely low calories, start adding calories back into your days, 150-200kcal at a time. So if you're eating 800/day (not saying you are), push it back up to 1,000 for a couple weeks, then 1,200 for a couple more, then 1,350, then maybe 1,500 after a couple months of adjusting diet. If you're eating more reasonably and maintaining bodyweight, then reduce calories in the same manner: if you're currently eating 2,000/day, go down to 1,800 for a month and see if anything changes. If it doesn't, go down to 1,600 for the next month and again look for any changes. Losing a couple pounds per month isn't huge on the scale, but it can mean a lot for actual body composition, appearance, shape and size if it's all fat.
The *if* of the above statement is where training comes in. How you eat will effect whether you gain/maintain/lose weight. How you train will effect where the weight changes come from. As a basic rule of thumb, for any improvements in body composition, strength training is a must, cardio is an optional addition. People normally get their training priorities the wrong way around when it comes to fat loss, because someone told them that cardio burns lots of calories and strength training makes you big. In reality, if you do a decent full body strength training session that focuses on compound, freeweight exercises (like squats, presses and pulls), you'll burn about the same amount of calories as you would in an intense cardio session of the same work duration. The training won't make you big, because diet has already determined whether you'll get bigger, smaller or stay the same weight and size. You cannot out-train your diet. What it will do is tell your body that your muscles (which will give you shape when the fat comes off) are important, so keep them. Thus the weight that does come off will be primarily fat, not muscle. Without good strength training, muscle mass isn't a prioritiy, so if you don't use it, you will lose it. Put another way, which is going to give you more visible abs: losing 10lb of fat, or losing 5lb of fat and 5lb of muscle? Obviously the former.
The other important factor in training is stress and recovery. Every time you train, it generates stress. Training can also relieve certain stresses, but it does generate physical stress which the body must recover from. Which do you think is the easier stress to recover from? 3x60min resistance training throughout the week, or 3x60min resistance training + 3x90min cardio + sports? Obviously the less work you put your body through, the easier it is to recover and the lower the physical stress levels will be. The more work you put your body through, the harder it'll be to recover and the greater the physical stress levels will be.
And stress causes hormones to do things you don't want them to do, such as hold onto bodyfat. As I mentioned before, your personality could also be a factor here: if you're highly strung and emphasise every little detail of your training and diet as critical, then your stress levels will be very high, and your body won't do what you want it to do.
Taking all this into consideration, do you think you should persist with doing unneccessary, stress-producing cardio during weightlifting class, and then doing weightlifting outside of class? Or should you just lift weights in class, maybe do 10-15min of conditioning to keep up your cardiovascular fitness, and call it a day? Do you think you should run 9 miles a week to burn more calories, or should you relax and let yourself recover?
