I have the same problem! I have a fiancee who likes to eat whatever he wants, cheese burgers, chips with cheese, the works!
The problem with me is if I eat that stuff my butt grows two sizes in a week!
You really need to be firm with your decisions. If you know that weekends are hard for you, pack snacks and foods that are healthy and bring them with you when you go out.
Also if you just HAVE to eat out and you have to eat something bad for you, get a to-go box right when your food comes and put half away so you don't eat the whole plate.
Especially when I go out with groups of friends it's very hard for me because I'm 25 and all of my friends eat cheese burgers and drink beer all day. If I decide to have a beer or cookies or whatever I make sure to tell myself that I HAVE to work it off later. So I make sure to go to bed an hour earlier so I can get up and work out in the morning and then again in the evening the next day.
I think the most important thing is to be firm with your friends! My friends do the same thing, they will say things like "what? you're not drinking, why not....just have one!!"
It's very hard to say no but you have to remember what you're working for.
I've started saying "No thanks, I've just got really good will power today and I don't want any."
