Perhaps the focus on the email snooping masks the inner shame. Yet, no one deserves to be treated like crud. Roller coaster relationships aren't worth it.
And I'll give you the reality/low down on the personality disorder mention[staying, perhaps message], unless they are willing to seek treatment, it won't do much good. You'll learn about your own reactions, probably will learn alot about yourself, your role, how you got there, but....unless they are 'willfully' in therapy [not the boundaries of you go or i need to walk]...it doesn't matter if it's just the drugs or if it's the underlying cluster b personality disorder...