I actually suggested to beth we end it, or I go find another partner that will meet my needs. She wouldn't accept that. She says she needs both of us, that Mark and I together fill her relationship needs and access different parts of her.
YOU are meeting HER needs.
MARK is meeting HER needs.
But Beth is not the only player here. (Only Beth's needs are mostly met at this time in inappropriate, dishonest ways) is not (You, Beth AND Mark's needs are mostly met at this time in appropriate, honest ways.)
But I know I need to just decide what it is I want. If this doesn't work for me, I need to be an adult and say so and work on finding what I need.
Yup. If participating in a cheating affair with Beth behind Mark's back in front of him while calling it "polyship" is not your cup of tea? Could stop doing it. Deal with it in appropriate ways.
If staying in a marriage that no longer fits is not your cup of tea? Could stop being married. Deal with it in appropriate ways.
Take care of your business, heal, and then allow yourself to move on. Hopefully to something honest and more fulfulling/satisfying than this.
Hang in there. You can do it!