Sorry to hear that Sarah.
The problem is after losing so much weight, you feel this false sense of "You've done so well and the job is now over"
But you've got to remind yourself of your goal - the bad thing is to feel depressed about not making progress.
It's kind of like if you worry about being nervous you'll be nervous... If you just throw yourself at it, you'll achieve your goal.
You've realised that you're finding it hard to get back on track, you've just got to say "TODAY, NOW" not tomorrow. When you start post-poning things you'll get into a habit of it, and that's what happens over christmas.
"After Christmas, I'll get back into everything" when really what you're saying is "I feel like stopping"
It has taken me almost 2 years after losing a lot of weight to really get back to achieving what I've always wanted.
Don't give into temptation - constantly remind yourself that exercise and good healthy food is your life at the moment, you cannot live like "normal people"
This is what I'm trying to keep up myself now.
Good luck, you can do it! You've achieved so much, but you're halfway to your goal - you're not done yet, keep going!