Trigger: something IRL that sets you off, all out of proportion to what actually happened.

I gather that the idea is to search back into the past to find where you experienced that feeling before, so you can trace it forward to its today-effects.

It seems that until that work is done, the trigger will not lose its power. But what if you can't find the memory of the event that caused similar feelings? How do you go about looking for it? I have one subject that's powerful enough to put me in tears at once, and I have no idea what it goes back to, have had no success in trying to trace it back.