people write out things just to let off steam and don't really mean them to the extent it seems. Open the lines of communication.

you got to talk about it... or you will just invent possible scenarios

they might just be venting their emotions and trying to gain perspective. this writing probably doesn't represent the entire scope of their feelings toward you.

they can demolish peoples' personal privacy and safety issues. You give no details, but I'm telling you, you've accidentally or not, changed the scope of your relationship forever. If it really was an accident, that's too bad, it sucks in fact. If you, like many former friends of mine, like to snoop through their boyfriend's and even ex-boyfriend's houses, invade their privacy and create drama, then I say you got what you had coming.

I'd never reveal that I read it. It was probably a way for that person to work through some acute feelings and doesn't represent the deeper relationship.

However, I would take note of the issues revealed in the letter, and I would try to fix whatever I could without being overt about it. You have a chance to make your partner happy and now you know exactly how to do it. Lots of people would love to have that kind of "insider info". Try to see this as an opportunity, not a problem.