Like Sassia said, it has to work, otherwise nobody would ever reach their targets.
There is a lady in my group who only had 1.5 stone to lose, the same as you, and she managed it fine. She had a huge loss of about 7lb the first week, and then steady 1-2lb a week losses until she reached her target (please dont summise that you will be the same, as everyone is different). All in all, I think it took her around 15 weeks.
It just shows it can be done, no matter how much you have to lose xx