Trying to lose 100 lbs, what's happening!?!?!?!?!?!?!

I started my weight loss journey in September of 2008 weighing in at an embarrassing 225 pounds. I'm 27 years old and I'm 5'1'' tall. I wouldn't say I was slender in my younger years, but before kids, I managed to stay between 120 & 140. I've had 2 kids in the last 5 years, and my husband and I have decided that we are done. So I finally made myself start getting into shape. I gave myself a weight lost goal of 100 lbs. To make a long story short. I started my workout with 2 mile walks at about 3.5 miles an hour. After losing about 20 pounds, I added in a high intensity step aerobic for an hour per day. After losing another 20 pounds, doing both of these exercises together 7 days a week, I added in a 16 minute Ab workout everyday and a 1 hour trip to the gym once a week. By Christmas 2008 I had lost 50 lbs. Weighing in at 175. After Christmas I continued with this high intensity routine that I made up (walk 2 miles per day, 1 hour of high intensity step aerobics, 16 minutes of AB workout, and a 1 hour trip to the gym once a week). By mid January I hadn't lost any weight at all!!!! The scale was officially stuck on 175! I had heard of plateauing duing weightloss, so I changed up my workouts,  and doing them even more intensely and I added in running stairs for 30 minutes per day (2 /15 minute session). As of today I'm pretty much still at 175! The scale has been fluctuating between 170 and 175 from day to day. Which I expect a fluctuation, but I can't seem to get out of the rut I'm in. I'm sure you are wondering what have I been eating. Basically I eat what I want when I want, but I make really smart choices about the types of food I eat. I do allow myself one meal of fast food per week, usually on Friday night because we have family night, and usually one sweet per week (a piece of cake, ice cream...) Typical breakfast is 2 pieces of fruit, a bottle of water, and a bowl of bran cereal, Lunch is maybe a plain turkey sanwich, or soup, and atleast one piece of fruit, Dinner is usually some type of baked meat, like chicken or porkchops (lean as possible) a vegetable, and a starch, usually rice, sometimes mashed potatoes. I eat 3-4 snack per day, usualy being fruit, sometimes a granola or protein bar. I take portion control very seriously so I measure out everything that eat. I make sure my main meals don't go over 350 calories, snacks- no more than 200 calories. I usually end up with a average calorie count of about 1500-1800 calories per day. And I've estimated that I'm burning about 2,887 calories per day. Sometimes I calorie shift if I'm feeling particulary heavy that week so calories my days would avg this : 900, 1200, 1500, 700, 1800, 1000, 800