I do that a lot, too. I'll be carrying on as normal, and then I'll hear or see something, often something that recalls a painful event or feeling, and then I'll just spiral. It always takes at least a few hours to climb back out of it; often I have to go to bed and then wake up in the morning no longer feeling sad. It's awful to feel like you're not in control of your emotions. It's like you're not even a grown adult, in that you feel like you can't cope with just the basic day-to-day banalities of life. I don't have any advice on how to handle it except to say I can empathize with that.