My last straw can be a myriad of things. But usually its when I finally decide i don't care what the repercussions are I will do what needs to be done.
I think most of us jail ourselves in fear of something we feel that we can't handle. Like if we tell someone what we think or put our foot down they will leave us, or argue and demean.
i've tried to get better about this because I will take a lot of certain people's crap, not all people (have no idea why). I don't like conflict is another issue - i will try and avoid it. Again I don't know why because in situations that merit it I can argue like no other, and I do not back down.
Last straw and action on it comes when I don't care anymore. When I can't take one more abusive, yelling, demanding, childish whatever. I wish I could tell you how to get there.