The one thing that has really helped me in my weight struggles has been embracing the concept of planned eating.
What is planned eating?
Just what it says...if you didn't plan on eating it, then don't eat it. This has helped me stay away from tempting, surprise sources of food that I didn't count on. This could be anything, such as free lunch, or a free snack at the office you weren't planning on being given.
At the beginning of the day, I map out what I am going to eat that day. I eat breakfast at home. Then, I pack my food for the day. It's a lot of food, too. It has to last me about 12 hours, so it's really 2 meals and a couple snacks rolled into one.
Then, if it's not in that lunch cooler, I do not eat it. This is hard work, because temptation is all around us, especially this time of year. Holiday meals, extra treats, homemade baked goods, etc. are all over the office. But politely declining doesn't usually cause too much lasting damage between friends and co-workers.
Occasionally, you will run across what I call a "force feeder," which is someone who no matter how many times you nicely decline their offer for free food, doesn't give up. They mean well, but their persistent attempts at sabotage seem to know no limits. Statements like "come on, just a little won't kill you." are the norm.
But my new comeback will be "sorry, it's not in the plan."