Same thing happens to me. There is always something, a party, a communion, wedding, or a "hey let's hang out and we can order pizza and wings and potato wedges" get-together, etc. I struggle the most with these gatherings too.
My advice to you is: try to hold a water bottle or large glass of water in your hands at all times. Keep sipping it (refill when empty)and if there are healthy choices, such as a vegetable tray or a tossed garden salad, fill your plate with that first, rather than the lasagna or pizza. It works most times and it works a bit better during the summer months when everyone is outside with a beer in their hands and you with your bottle (of water) too.