CS, if you have that much to lose you really need something you can stick with for a long time. If you lose weight too fast it'll just go back on really fast, so a gradual diminishing is what's best. You'll also find it easier with support, because most people it seems hit various plateaus (should be plateaux, I know!) when the weight loss either gets stuck or even runs backwards into slight gains, and that's when it's really hard to stay motivated.

Good luck! But PM me if you would like more support, and/or if you'd like to know more about Slimming World (which I really do recommend).