how do you resist snacking when surrounded by it!!

i've had a rubbish few days calorie wise. was doing really well, and still am in terms of my meals, I have them all planned out and they are filling and low calorie etc. BUT with it being the school summer holidays we are going to my mums, sisters, friends houses etc all the time and everyone gets out bowls of crisps, chocolate, sweets, then theyre doing buffet food for lunches etc.
I'm taking my own lunch and sticking to that but i;m nibbling on crisps, stealing a chicken nugget off DD's buffet plate, a piece of chocolate here and there, and then when I get home and log it I've managed to "nibble" 400-500 kcal! a handful of crisps is like 150kcal! I'm not eating because I'm hungry, I'm not, I;m eating because its there infront of me, everyone else is eating it and damn it tastes good!!
trying so hard to be healthy and these little momentary lapses are killing me!!
i've tried chewing gum and then I can't eat it, but I can't chew it all day!!
this is the 3rd day where i've been p###ed off with myself when I get home and realise i have like 100kcal left for tea so am blatantly going to go over today AGAIN!!!