You're not doing enough to lose weight. What is your exercise? If you run a mile every day that's not enough. Run for set periods of time rather than distance. Go for 20-30 minutes every day. And don't just run. Do other exercises as well. These include burpees, jump-rope, P90X; whatever. Intense aerobic exercise that keeps your body temperature up and your heart racing fast are what will make you lose weight.
Diet: What are you eating? You mentioned diet soda and craving more soda. Stay as far away as humanly possible from diet sodas!!!!!!!!!!!!!!!! I can not stress this enough. I will repeat: Stay as far away as humanly possible from diet sodas!!!!!!!!!!!!!!!![/u
Get it? If not, then:
Stay as far away as humanly possible from diet sodas!!!!!!!!!!!!!!!![/u
But why? Aren't diet sodas good for you? Diet sodas have no sugar! Well they are still "sweet". This sweetness comes from something called "aspartame". Aspartame is an artificial sweetener that triggers your body to produce insulin. Insulin is what your body uses to store fat. It stores fat by taking sugar away from your blood, which causes low blood sugar. Low blood sugar causes you to have low energy, which will not only make you skimp your workout, but also make you hungry. Being hungry will make you want to eat. Eating will make you fat. Additionally, because aspartame is sweet, your body just prepared to digest something sweet. When you don't get that sugar, your body basically throws a tantrum and demands you eat something sweet. This causes cravings for sugary food which could be exactly why you aren't able to kick your 'addiction'.
As far as diet is concerned, eat things low in calories. Carbohydrates, fats and proteins are essential to your diet. Avoid trans fats and saturated fats. Carbs will provide your body with energy to expend during your workout. A serving of potatoes or rice will do fine. Bananas are great too.
To lose weight, you want to burn more calories than you take in a day. At 170 pounds you will need approximately 2380 calories just to maintain your weight. Therefore, in addition to exercise, try consuming around 1700-1800 calories a day.
And please, for the love of God, don't be one of those people who uses the retarded-holy-s**t-how-are-they-even-open-still-people-are-retarded-for-going-here Planet Fitness logic and reward yourself for a workout with something like pizza, cupcakes or something sugary like that. Planet Fitness logic is to keep you fat so you keep throwing money at them to use their facilities. Don't use that logic.
Edit: And contrary to what the person above me said, fat is not good. You do not build muscle on top of that. That is not physically possible. Fat is just fat.