I think you either have to confess and try to work through the situation or decide to leave the relationship and do so.

You do not know the context of what you read, it was not addressed to "you" and, though it appears to be about you, you do not know if your bf was using referencing you as an easy out for something else or if he truly feels/believes what he wrote and/or where he stands with leaving the relationship himself.

You either have to confess your reading something not addressed to you and further curiosity about the context; let your thoughts eat you up; decide to go with your thoughts and leave the relationship.