It doesn't have to be mango lassi---it can just be regular sweet lassi.
Or if you don't like plain sweet lassi, just get yogurt in your diet some other way---raita, for example. Either way you will get the benefit of the healthy yogurt cultures in your digestive system...
Whenever I'm trying to resist some of my foods, i just tell myself, "This food will still be here in two months after I get married. But for now, it's worth it to resist so I'll look my best on the big day." Just take it one day at a time and realize that you will be able to enjoy these foods again after your big day!