Hi Jayne, welcome aboard! Losing 20kg by christmas is a BIG goal - losing 1-2kg per month is usually considered the wisest aim when you are aiming for "maintainable" weight loss. It's great to have high aims but it might be good to break it down into 5kg minigoals along the way so you can reward yourself as you go?
Congratulations on the upcoming wedding
I'm sure you will be a beautiful bride!