I would love to know what causes this too. I haven't yet gotten an answer from an English speaking dentist. So personally i think they don't know. IF it is stress i've found no difference between when i'm having the best week ever and when my dog died and my truck got stolen and my baby left me etc etc etc.
Something to explore when you do see a dentist, I was given an NTI no idea what that stands for. but it's a mouth guard that fits over about my front 4 teeth on the bottom and it's slanted the idea is that the top teeth touch this and spring back open due to the sloped surface. I haven't had great luck with it because i am such a super severe grinder but maybe it would be something for you to check inoto.
As to your question, I would break off any loose plastic and probably wear it more for comfort, unless you can get a do it yourselfer from the drug store.
Does your dentist not have a colleauge that covers emergencies??? because i would actually consider this to be an emergency because you might end up cracking that tooth.