If you want to boil weight loss down to a single problem, that single problem is that your body is a very complicated biological machine. While nutritional science and biological knowledge of digestion can create plans that work for 90% of people, there's that niggling 10% that don't respond to the plan the same way a normal person does. Losing weight isn't just about changing the food you eat. It's not just about changing your lethargic lifestyle. It's a combination of several factors, and a process of becoming more aware of how your body works.
Take me for example. Processed food diet, 4 cokes/day for 10 years or more. Perfect blood glucose. Perfect blood pressure. Triglycerides so high I'm guaranteed to get pancreatitis within 10 years if I don't correct them. Low HDL cholesterol, and LDL can't be measured due to error introduced by the triglycerides.
So I'm on a South Beach diet with a prescribed 40 minutes of aerobic exercise 3 times weekly, more if possible. This diet is well-tuned to my needs actually. Not all carbs are good: simple carbs and sugar carbs digest very quickly and spike your blood glucose. That causes the pancreas to spew out insulin like crazy. Too much insulin gets put out, and glucose drops. Then you're hungry again. Ever had that drag around 3-4 where you need a snack? That's the cause. "Sushi coma" is what happens when you dump a cup of highly processed white rice into your stomach (it's like eating a handful of sugar since the fiber's been bleached and processed out). Sugar makes glucose, insulin eats glucose, and when glucose drops too low your brain triggers hunger cravings to force you to eat something that will get more glucose in you. In the end, you should favor as few carbs as you can get away with while maximizing dietary fiber. The stomach processes carbs from fiber more slowly, and the insulin response is more healthy.
Fat's similar. Trans fat is horrible for you and should be avoided completely. Saturated fats are bad as well and should be minimized. Unsaturated fats can be healthy for you; in particular Omega-3 is a polyunsaturated fat that *smashes* triglycerides. In the end, you don't have to watch the *amount* of fat you intake so much as you watch the *kind* of fat you intake.
Anyway, the point is arguing about carbs and fat usually focuses on numbers, when source is important. Sashimi is a great food. Sushi not so much. It tends to come bundled with a ton of white rice, and around here it comes with fatty sauces. If you could get it with brown rice (which might make the sushi chef attack you) it would be better due to the increased dietary fiber. I just ate some brown rice jambalaya with turkey sausage and it's as good or better than any other jambalaya I've ever had. I've got some frozen pizzas with a flaxseed crust and skim mozzerella that are healthier for me than a salad, because it has lots of fiber and Omega-3. I can also trick my body into making meals healthier than they are. A glass of Metamucil 15 minutes before a meal means I eat less and have a lot of fiber that slows down processing of simple carbs. Tomato sauces are acidic, which slows down processing of simple carbs. So does water with lemon.
Exercise is good, but if weight loss is your goal you really don't want to build muscle. I take that back; if weight loss is your only goal you need a better reason to diet. Muscle weighs more than fat. If you build muscle while you lose fat, you will likely see an increase in weight. But you'll be more healthy! Muscles burn calories even at rest. If you have more muscles, you can consume more food in a day without contributing to your fat reserves. I've heard that Michael Phelps has something like a 33,000 calorie diet; but that guy swims 5 miles like you open bags of potato chips. If the average person needs 1,500 calories per day, I'd wager Phelps probably needs 2,800 or more due to his high metabolism and muscle demand.
So what I'm getting at is weight loss is kind of a silly goal; health is a better goal. If you change what you eat, your heart health will be better. If you do aerobic exercise, your cardiovascular system and respiratory system will be better. If you build muscle, your metabolism will make you more resistant to falling off the diet from time to time. Focusing on any one of these things at the expense of others seems silly.
I'm not just blowing smoke here, either. Last year, the only vegetables I ate were fried potatoes. Now I eat salads. I used to eat a bacon cheeseburger plain for lunch 3/5 days of the work week. Now it's a turkey burger with half the whole wheat bun tossed in the garbage, topped with lettuce, a tomato, and red onion. I also eat salads from time to time. I drink at most one coke per day, and that's maybe 3 days a week. I either play DDR or jog on the treadmill for 40 minutes at least twice a week (though sickness and other factors have kept me away for a couple of weeks.) The goal is to feel my heart pumping, sweat, and get out of breath and maintain it for as long as possible; this burns fat.
I lost 20 pounds in 2 weeks when I couldn't even spend more than 20 minutes on the treadmill at jogging speed. I've maintained -25 pounds for 4 months now, and I've slipped *far* off of the diet in the past month. I can see my ribs through my chest again. My face is more thin. I don't lose my breath as easy. Unfortunately, despite all this my triglycerides are still super-high despite a cholesterol medicine and ridiculous doses of fish oil. My doctor thinks I'm skipping doses. My guess is I have a genetic predisposition towards high triglyceride production, and I'm going to have to fight harder than others to see meaningful progress. But I can either choose to alter my diet and lifestyle now and enjoy the process, or I can develop pancreatitis followed by Type 2 diabetes and have the choice made for me. A man chooses, a slave obeys.
My advice for you? Find a diet plan that you can follow. Read a book about it. Make sure it's backed by science. Learn the science. Then find a way to incorporate those rules into your life. I don't worry about what I eat for more than a moment or two anymore, because I learned what to avoid. I don't think about exercising because it's part of morning routine and makes me feel good during the day. I don't miss old treasured foods because I let myself eat them once every two weeks or so. I don't miss prized condiments like sour cream because I've learned that the burrito tastes fine without it. If it feels like you're losing something, you won't stick with the plan. If it feels like you didn't really make any changes, you'll stick with it.