Just get out and have other polyamorous relationships. Telling Mark will only cause more pain. Maybe, I'm time, you can find a "primary" style relationship and Beth and Mark will decide poly work for them too and turn you and Beth can have a "secondary" style relationship. Maybe Beth will decide to be with you in a polyamorous relationship and sacrifice Mark in order to do so. Maybe Beth will stay with Mark in a monogamous marriage and you'll shift to a loving friendship with her that doesn't cross their monogamous boundaries but allows you to be in her life for the long term.
You can keep those scenarios as real possibilities by making sure you don't let this get ugly and to the point where it isn't healthy to interact on any level