I think this type of pattern is REALLY common.
The key is pretty simply really - lifestyle changes that you can maintain, as opposed to going on a "diet".
Once you deprive yourself of something, you CRAVE it. Go on a strict diet for a week, and when you blow it, you will BLOW it!
I think you should start with small things, small goals.
Set a goal to do some exercise for 20-30 minutes, 3 times a week. When you have achieved this for a couple of weeks, step it up.
Set a goal to drink 2-3L of water a day. When this is habit, add in something else. Watch your portion sizes, but keep it realistic. Limit takeaway food. Limit after dinner snacking. Keep healthy snacks in the house.
Go at YOUR OWN PACE.
Does it really matter if it takes a year or even more to lose the weight, if it means you will keep it off?
Aim for a loss of 1-2kg a month. Doesn't sound like much, but trust me, its the easiest way to maintain.
Best thing is - in the end, you are healthy, and living a healthy lifestyle. There is no "coming off" the diet, because you have just made healthy changes for the better!
Don't beat yourself up for slip ups. Take a deep breath, and realise we all do it, and move on. If you are DESPERATE for chocolate - have a small piece, savour it, and get over it. If you deny yourself for too long, you'll end up eating a whole family sized block!
Best of luck, hope my advice helps!