I think I experience exactly what you describe. I'll be feeling stressed out, either by something external or something internal, like anxiety and depression, and during those times I feel like I'm always on a edge, and about to fall off at any time. Then some relatively small thing can cause me to lose it, but it's part of a larger feeling, that lasts for days or weeks.
I usually get stressed out by situations that I find impossible to cope with (rather than individual instances), and those will last for several weeks or even months and then I'm in an ongoing near-meltdown mode.