muser, can so relate to your trust issues. idk but therapy helped me deal with this a lot. some of the solution is setting boundaries with others. then if trust is broken we need to assess if the offense is (1.) totally unacceptable or (2.) if we are willing to accept what happened-we all are not perfect-...(that doesn't mean either to forget the transgression 'cause we learn from things) and try to rebuild the relationship. i was told to be cautious with the offender after that if i chose to continue the relationship. that doesn't mean being paranoid, just careful to protect myself and feelings.
does this make sense? hard to write a short post sometimes.
as for ruminating on it...that keeps you in the past...don't i know! have u discussed this with the other person? sometimes that gives clarity and also allows us to express how it made us feel. then hopefully one can build on that with the other person.