Co-worker liability for invasion of privacy

I don't believe there is any legal recourse you can take. There is no legal protection from having someone circulate a letter or memo that one writes oneself. If the letter had been at your home, there might be a cause of action for the coworker removing something from your home--burglary, for example. But assuming the letter was at work, an employee does not have any privacy interest or rights in their work space.

At the risk of pointing out the obvious, if you don't want it spread around, do not say, write, annotate, email, document, etc. it. By documenting something sensitive, you placed yourself at risk.

While this is unfortunate and highly unprofessional, it is not unlawful behavior.  The invasion of privacy is quite specific as to what constitute an actionable offense, and it appears on the facts presented that none of the elements of this tort have been met.  An employee has no privacy rights with respect to their work space. 

Possibly if what was told was untrue there could be an action for defamation.  However, based on the facts given, this is not the case here. 

Unless this conduct breached a stated company policy, violated a union/employment contract, or constituted some form of discrimination/retaliation,  there seems to have been no illegality in this case.