when enough is enough

What is the most you can handle? When someone is testing your limits how do you know when to throw in the towel and say enough is enough? I'm to that point of questioning how much more am I supposed to endure. And do I really have to? I know dealing and getting frustrated is my own doing. I just keep looking at that light at the end of the tunnel. But it's fading. Or maybe it's not what I thought it would be. I would just like to know what the right thing to do is so I can just do it. And know it's for the better and that my efforts will be rewarded in some way.