I am confused on what I should do. The partner of my partner does not like me. When M. and I have plans, she make it a point to have a fight with him.
Has she told you she doesn't like you? Has she told someone else she doesn't like you? Do you know for a fact that she is fighting with him
because you and he have plans? And you believe she does so in order to....? Accomplish what?
I know that there is someone in my life who is forever talking about other people--including me--and informing the rest of the world how so and so feels, and why so and so behaved as they did...and this person is a million miles off base and just flat out wrong about other people's real feelings and motivations. And in my own life, I have, plenty of times, believed a person acted out of [fill in the blank] and later found out it had nothing to do with that. They weren't mad at me, for instance...they just found out their son had cancer....or whatever.
I am finding it complicated to communicate with him as she assures him that she is not upset by my presence.
Maybe she isn't. Maybe there are other stressors in her life. Or maybe she is, but she aspires not to be, and would like not to be. Is she someone who was poly to begin with? Was she in his life before you or did she meet him after you and he were a couple? Is she trying to 'be poly' when she had no interest in it, simply to be with
him? Perhaps she's like many here, struggling to overcome jealousy, and wishing she didn't feel that way?
I do not care who he has relationships with, but he tries so hard to make her happy and I am suffering. I have tried to be tactful about telling him that I am being negatively impacted by their relationship.
What specific actions of his are hurting you? It might help to be able to give him concrete items to work with, rather than a vague, 'your relationship with her hurts me.'
...since she moved in with him he has been less and less involved in my life and it does hurt me. Does anyone have any idea how to communicate with him without making me sound bitter?
"You've been less involved in my life lately, and it hurts." This doesn't sound bitter at all. "I miss our time together." That's not bitter. It's telling him how you feel about the two of you; it's telling him you enjoyed and valued your time with him.
Best of luck.