really slow weight loss...please advise!

I have been on my diet, or "new lifestyle" since November and have lost 21 pounds...that's roughly 3 lbs a month...considering I still have 50 lbs to lose, that is NOT good enough! Anyone have any tips for what I should be doing differently? I typically eat between 1200-1700 calories (varying it daily) and go to the gym 4-6 days a week. i usually do about 30-45 minutes of cardio when i'm there, burning (according to my heart rate monitor) between 275-450 calories depending on what i do. I also strength train 2-3 times a week. I eat back most of my exercise calories...I am lost here...