Lots of baby weight to lose :(

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Hi, UK mummy of 2 needing to shed some serious baby weight. I put a lot of weight on with my first baby (born July 2010) but managed to lose it all (and an extra stone) without any real effort (put this down to breastfeeding and being more active after a lazy pregnancy). I had my second baby in June and I put all the weight back on again but this time, I have literally only lost the weight of the baby and she's nearly 12 weeks old now :(
I am exclusively breastfeeding and have used it as an excuse to eat junk, I'm a total sugar addict and have been pretty lazy since I had my second but I'm desperate to shift this flab, I don't want to be wearing maternity jeans forever!