Ok so stop panicking for a start. Finished? right. Let's get cracking.
The first thing you need to do is get a picture of your wedding dress and stick it to your computer screen at work (if you have one), on the fridge at home, on the dash board in the car....you get my drift.
The next thing you need to do is get rid of all the the sweets and chocolate and crisps from your house. If you can't throw them away (and you should! At least in the beginning) then put them all into a carrier bag and put them away out of sight (I have mine right away on the top of my kitchen cupboards!).
Then you will need go and buy some free snack foods - be imaginative are you a savoury or a sweet person? If you are sweet then buy cherries and pineapple and grapes and have it all washed in bowls in the fridge ready for when you want to snack. If you are savoury then get some pickles or olives (check the syns) or make some slimming world hummus/mackerel pate and cut up some carrots and do the same - have them ready to go in the fridge waiting for you to snack! Preparation is EVERYTHING. So plan your meals, try and snack mostly on fruit and veg, count your syns and most importantly never ever give up!
I completely understand what you are going through as I did it myself last year - I bought a dress that didn't fit and had to diet my way into it. It was hard but i did it because there was NO OTHER OPTION!
You can do it too. You just have to get your game face on and keep your eye on that prize - i.e. your wedding day, feeling amazing, having clear skin and bright eyes but also being able to look at the pictures and think "Yes! I looked HOT!".
Good luck!