Originally Posted by LloydN
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.
Um yeah, now Beth needs her ass kicked.
There is more to this relationship than Beth getting her needs met. There are Mark's needs (although at this point that is between them) and there are your needs as well. It would be a good exercise to determine what your needs are, which ones she is meeting, and which ones she is not. Then inform her. She can choose to address those unmet needs in an authentic way or she may come to the conclusion that she is unwilling or incapable. If it is the latter, then you have every right to get those unmet needs fulfilled elsewhere - whether it means including some one new in your poly configuration (although I beg you not to do this until you have your shit sorted - doing it before the shit is sorted is an easy way to hurt yet another person), or breaking up with Beth and starting anew.
While it does make one feel wanted to be needed as when Beth stated she needs you both, you are not a toy she can play with as the mood and situation permits. She needs to ask herself what it would feel like to be in your shoes. Heck you could ask her if she thinks she would be happy with your personal situation.
Good luck to you!