I wonder, is it a matter of priorities?
First, I always gain a bit of fat during the holidays. Why? Because I'm around more sugar and it is almost impossible to NOT eat at least some of it. Also, my body seems to want to retain fat more easily during colder months.
But I loose motivation when I loose focus on my goals. That's usually because there are other things going on in my life (job, relationship, life difficulties) that just take too much time and energy (emotional or physical). There are times when I just have to say "fuck it" (FOR NOW), take care of what needs to be taken care of, promising myself that I *will* get back on track once I get through whatever the problem is.
This is different from giving up; it is consciously shifting priorities, but only temporarily.