I can't seem to lose weight - despite a vigorus workout routine!

I am a 27 year old female and about 40 pounds overweight, and have been trying to lose weight my whole life it seems. I just got married, and felt a new desire/determination to try again. So, for the last two months I have been hitting the gym for 1 hour 5 days a week. I do circuit weight lifting twice a week, take 2 spinning classes a week, and then add in another varying type of cardio the 5th day of the week - I'm pushing & challenging myself every time! I have cut back on my eating, and try to make wise choices (not eating fast food, pizza, desserts, etc.). My typical daily meals are cereal for breakfast, a sandwich for lunch, and a reasonable dinner, snacking lightly between meals on fruit, or string cheese, or something like that.

All of that to say that I'm not losing weight!! I can't figure it out! I gain muscle really fast, and I can feel that I'm building muscle, but I'm not dropping pants sizes and not droping weight on the scale. It's so frustrating!!! Am I working out too much?? Do I need to do all cardio? Am I eating wrong??

HELP!!!!