It's always good to aim high, but the reason why many weight loss efforts fail (in fact, 90% of those dieting will put the weight back on - and then some.. hows that for a scary statistic!!) is because people go too hard - lose weight really fast with very restrictive diets and massive amounts of exercise.
Then, as that is not really sustainable, when they return to regular patterns of eating, their "starved" metabolism is still just as slow as before - maybe more so if you were more focussed on restrictive eating rather than exercise, and you gain the weight back.
The idea behind slow weight loss of 0.5-1kg per week is that you are not on a diet, and you are just making lifestyle changes - healthy eating habits, the odd reward (it's what you eat 80% of the time that counts) and regular exercise. The exercise speeds up your metabolism and ensures you don't end up with saggy skin.
After this, you need to be on maintenance - so continuing to eat healthily (hopefully by now, a habit) and doing regular exercise, and that is the way to keep weight under control.
If you have a look in our maintenance section, you'll see some inspirational girlies who lost a lot of weight - slowly - but have now maintained their new lifestyles and weights for months! (see zeedevelgirl, butterfly_dawn, and everyone else there!)