Perhaps the worst part about it is that in my head, I always work in ideals. Ideally, everybody I invite to a party will tell me whether or not they are going, and bring what I need them to if they offer it. I can plan for possible counterpoints in my argument, but it's impossible to know exactly what they're going to say when that conversation actually does happen.
I think what's strange about this is that I don't consider myself a very flexible person. It's sort of weird to think that way, since I'm an improv coach, but acting is different than real world scenarios. I can plan for things to go wrong, but I can't plan for what things will go wrong. Most often the way I try to handle this is by preparing everything and making backups that will solve most problems that arise. For example, I'll bring copious amounts of things to do at the party so everybody has a bunch of options, and I'll make sure to have money and transportation on me, just in case anything goes wrong.
This doesn't work for things like conversations, of course. I know how to steer conversations, but I can't do it under pressure. Most often, if something goes 'wrong' in any situation and I'm not prepared for it, I won't know how to handle it, and thus it won't get handled.
I think the way to go about ensuring things always work out is to teach yourself how to adapt. Maybe this is best done in practice, I don't know, but growing more confident in one's actions and preparing to be caught off guard will help things to run more smoothly. Trust people with as few variables as possible, and leave the rest in your control. In a potluck, make other people bring food while you bring the cups and silverware. You can trust yourself to bring them, but that alone is a larger responsibility than any one food, even if it is less work. If that person messes up, everything fails
So put yourself in as much power as possible, and don't try to plan for what will go wrong. Just be generally prepared for the fact that things will go wrong. Some things go wrong more often than others, and you can learn from that experience, but don't try to cover up possible mistakes that are unlikely to occur just because those mistakes happened in the past.
Basically, trust yourself and nobody else.