Need to vent!

I'm usually not one to post on message boards, but its my desperate attempt to see if anyone has gone through the same thing and may have some advice.
At the beginning of May, my relationship turned into a long-distance one, he now in Windsor, Ontario, me still in Chicago. We have an amazing relationship and he has been nothing but supportive during my weight loss journey, even choosing fitness goals for himself to do along with me. However, the distance has been causing massive setbacks for me. We see each other every two weeks or so, switching off who travels and when he comes here, I am generally able to stay on my weight loss goals. However, when I visit him, I always gain back several lbs which takes the whole month to lose over again and by that point, I'm heading back to Canada.
Part of the issue comes from his mother who is so gracious in making us dinner or lunch every time I'm traveling through Michigan, but she is always putting more food on your plate or gives you desserts and takes great offense when you don't take more or when you don't eat all of it. This is the biggest part of my setback.
Then, since we don't go on dates and neither of us really go out when the other is not around, we go out to nice meals often and do fun things. I still make healthy choices in what I eat, for example choosing the not so fabulous sounding simple fish dish instead of the probably caloric beef and chicken kabob. We also try and get at least one workout in during our trips which usually last three or four days. This is still not enough! I manage to always gain back 3 or 4 lbs.
I've been stuck at this weight plateau for three months because of it. So frustrated! It's causing a lack of motivation on my part.
Anyone been in a similar situation?