Boy I wish I had answers for you. I'm actually just offering support. I also am an extreme introvert, and the inability to go somewhere that I don't feel wanted is causing a lot of problems for me, too. I'm currently working on my own solution and not feeling too hopeful.
I think it's great that you're allowing yourself to seek reassurance. It seems to me that polyfolk have a tendency to think that they shouldn't ever feel that way. You should also allow for the fact that, no matter how much you think ahead and analyze yourself, sometimes you just won't know how you feel until you are in the situation.
I don't know if it will help you, but I handle uncertainty by asking for rules. This drives some people crazy, but so far my loves have been very tolerant and it's reassuring to me because then I know that something won't change until they've talked to me about it. It's important that you make sure everyone knows the rules can be renegotiated if you go this route, and if you need them to be set for a certain amount of time, it's important that everyone know that too.