1. You are more than a number. This isn't meant to be some healthy at every size crap, but you have value and worth beyond a number. You are not defined by what you weigh or what you eat.
2. That is almost certainly a high proportion of water weight because it's not usually possible for the body to gain that much *actual* weight that quickly.
3. Progress, not perfection. How are you feeling? You're still down from your start and you mentioned early on how much better you were feeling.
4. Holidays are hard when trying to manage a particular diet. Have a plan. Office goodies - partake to your heart's desire or only allow yourself 1 grab a week? Or hold out for a specific goodie you know you love and will be there. Having a plan helps.
5. Holiday parties - take what you truly love to eat and enjoy it. Skip the so-so's or even eating "healthy" stuff just because you feel you should. Sure, eat some veggies to take the edge off but if you're still going to eat the other stuff anyway, might as well save yourself the calories.
Good luck, this is a rough time of year to be trying to change your mindset around food and exercise.