I think that happens on any weight loss plan, if you don't keep eating healthy and exercise when you come off it - you will gain. I've done it heaps of times, so my tip would be to work out something you can sustain for life. Sure while you are in the weight loss stages... you are going to be stricter on yourself, but you really need to think about your health as a whole and work towards changing your habits completely.
You don't have to be without chocolate - compromise and switch to Dark 70-80% cocoa chocolate. It's better for you!
It's pretty daunting to all of a sudden turn your way of life on its head. You might want to start out slowly by cutting back on things week by week. Like this week you might cut soda out of your diet and switch to water, next week you might cut margarine out of your diet... the week after that you might cut out white bread and switch to brown, the week after switch to dark chocolate. Simple stuff like that until you are used to your new style of eating.
Increase your vegetable intake - cut back on cheese.
Cook with good quality oils like olive oil.
Try to eat 5-6 times a day, every 2-3 hours. I'm not talking huge meals.... i mean Breakfast, Lunch and Dinner - and then 2-3 other smallish meals in between... fruit, nuts, low fat yogurt, vegetables with a healthy dip, a couple of slices of ham.
Eat protein .... Chicken, lean meats, fish, eggs, cottage cheese, tofu - I try to eat protein with breakfast, lunch and dinner.
Depending on your size, try to drink 2-3 litres of water a day.... but don't go and chug 1.5 litres in one sitting. You will probably just pee it out. You need to spread out your fluid intake evenly throughout the day so that your body can use it to your full advantage.
If you aren't keen on eating vegetables - buy yourself a good quality cold pressed juicer. Cold pressed juicers keep more of the nutrients in the juice because they crush the food instead of cutting it. Have a glass of fruit / vegetable juice every day.
Try not to eat about 3 hours before bed.
Get enough sleep!
Exercise - when i say exercise i mean do whatever you can do, even if that is 5 minutes a day. Some exercise is better than none. Try and find something you enjoy doing. Maybe you like swimming or playing WII fit / sports with the kids?
I'm fat so don't take my word as gospel lol. These are just some ideas i am throwing out there. I also find it helpful to keep a food and exercise journal, i can look back and see where i have gone wrong if I've had a bad week.
Celebrate your success! Heaps of times I've lost weight and never told anyone my start weight, how much i lost, or the weight i got to. Even if you only tell people on this forum of your success.... it truly does inspire and motivate you when someone tells you that you did a good job.
Talk about your struggles. Don't just bottle it up until it drives you crazy! lol
All the best!