Mental illness is not always constant. There are days, weeks, or months where you could be feeling on top of the world. You’re taking care of yourself, you’re getting things done, the days don’t weigh you down like they used to – momentarily, you feel like a real person. I think that’s one of the scariest parts for people with mental illness; the knowledge that the “real” feeling is only temporary, and that it’ll burn down in a ball of flames one day. And that day could be any day. And it’ll probably be terribly inconvenient.
Maybe crashing is mostly consistent with those who have bipolar disorder or PTSD, but I’d argue that the combination of depression + anxiety can create a very similar crash. The labels and technicalities aren’t important and I’m not a therapist. All I know is that crashing, at least for me, is serious. The higher the high, the lower the low. It’s chemical.