I think you're confusing "taking a strong stand" with "being a dikk." You can be friend-LY to her, treating her civilly in whatever interactions you do have, without being supplicating and needy.
It's possible to nicely say "Here, let me give you a hand with the rest of your stuff that I packed up for you while you were gone this weekend," help her load it into her truck, and send her pretty little adulterous ass on her merry way.
She will begin to respect you when YOU begin to respect YOURSELF. Not a moment sooner.
