Post-Pregnancy Weight Loss

I gave birth to a healthy baby girl 10 weeks ago and I am experiencing great difficulty in returning to my pre-baby weight. I currently weigh 11st and I wish to return to 9-9.5st. I have tried Weight Watchers, the Special K Kick Start Diet combined with Adios slimming tablets and to get below 11st is proving impossible. Needless to say I am also doing a combination of cardio and low-impact exercise approx 4 times a week for an hour minimum so lack of exercise is NOT an issue. I am also feeling the pressure as I am a bridesmaid on Sept 30th and my dress is a size 10!! Pre-baby I never experienced problems with weight-loss. Has my constitution changed due to child birth? Could someone please help as I am getting deperate and feeling very low.