Valentine's Day was just another day for me food-wise, but I've gone off plan so many other times I can't even count them all. Usually for something that is planned in advance, like your Valentine's Day dinner (dinner out with friends, my birthday, it's Friday and I need a glass of wine). Sometimes for a week at a time
(business trip, vacation, houseguests). One meal off-plan doesn't seem to have much of an impact on my weight. One week off-plan, that's another story. Here it is almost March and I'm still trying to lose the weight I put on over the holidays.
I've definitely experienced that it can be really hard to get back on track after being off-plan. Once I have a few good days, I'm usually fine, but getting in the first few days can be tough.
During one struggle, I finally just threw out the food that I had around the house tempting me. Another strategy that has helped is posting my meal plan for the day in the planning thread here in the Support forum: it forces me to plan in advance and I'm more likely to stick to my plan once I've posted it. When I was at my absolute lowest, another thing that helped was being accountable to someone else and telling that person what I was eating. In my case, it was my personal trainer, but it could be a friend or anyone. Knowing my trainer was going to ask me what I ate when I went in for training really helped me get back on track.