I've lost exactly seven stone since I started just over a year ago. Yes it is do-able, as long as you are totally focussed and prepared to stick at it.
Keep plenty of variety in your diet so you dont get bored, switch between plans to keep your metabolism on its toes and keep yourself challenged with different small goals to keep your motivation going.
Good luck!!
ETA - even 5 stone would be a massive difference in your body shape and dresses can be altered once you get to target. So dont panic if you dont quite make it, just keep focussed and you will definitely be there by the wedding!