My son is almost 10 and his dad and I got divorced when he was 4. He began dating a women I was slightly acquainted with ( she was the old girlfriend of a buddy of his) and we found out how serious it was when he announced he was moving to her condo and had to change his visitation schedule!
I actually invited her to meet me for coffee and asked what her expectations for my son were, now that he would be in her house every other weekend. I asked if he would have his own room, etc. and said that I wanted him to feel comfortable and safe there and that he 'knew the rules' at the new place.
She actually told me that she was a child of a very bad divorce and had not had a relationship with her own father for about 10 years growing up! She said my son seemed very happy and well-adjusted and that she just wanted to try to help keep things stable for him. As it turned out, she is MUCH more responsible than my ex!! So although we have all had our disagreements and rough patches, she has improved things in some ways.
It can be hard. You are both pre-disposed to be on opposite sides- the stepmom may have heard an earful about you from your ex and she is bound to take his side of things.
However- even if you don't agree on everything, can you agree that each in your own way, you BOTH have your daughter's best welfare, health and happiness at heart?
Do you have a Joint Parenting Agreement as part of your custody arrangement? Sitting down (all parents, natural and step) and talking about specific issues, holidays, problems, etc. with a good mediator has helped my ex, his wife, me and my fiancee work through some things that could easily have turned into giant earth-scorching arguments on our own.
you may not get everything the way you want it, but you will get a chance to speak your side, explain why you've made certain decisions about your daughter - and why you feel it is important for HER well-being that those decisions are respected and followed by both parents.
My experience has been that your ex might not take everything YOU say seriously (and by extension, his wife won't either) but if the mediator helps everyone to see both sides of an issue or why something is important, they will be more likely to respect the decision. In our case, she has also written up documentation of the consensus we've all reached on certain subjects and my attorney had them legally filed as part of our Joint Parenting Agreement.
If the mediator helps you out, it isn't a case of 'You SAID you would not tell her that!" or "we asked you not to let her watch that" or whatever. It is a decision you all came to TOGETHER, with the help of a professional witness. It makes it less 'personal' between you and the stepmom.
That said, we don't always see eye to eye. When my son was your daughter's age, he went through a phase of talking about his stepmom ALL THE TIME when he had seen her. I have to admit, it was sort of hurtful. But gradually I have come to realize that despite that talk- they are nowhere near as close as my son is with my fiancee and I. I am always the mom- no one can take that away and I don't need to feel threatened!
His relationship with his stepmom is sort of like she is an aunt that he really likes. I trust her to take good care of him when he is at their house and she and I are polite and pleasant to each other.
You do not have to be BEST FRIENDS- it is unlikely you ever will. You just have to be polite and practical and get along well enough to go to school events, sports, etc. If all sides can agree that it is the best thing for your daughter to be that way, at least in public and in front of her, it will get easier over time, trust me!!
If you have to argue or fight over things, do it when your daughter is not around and grit your teeth and smile when the step comes to pick her up. Your little girl will be under a lot less stress if you can both act like the grown-ups for her sake.
Good Luck!! Feel free to message me if I can give you any more specific advice about mediation. It is TOTALLY worth the money- less $$ than an attorney and most judges will order you to see a mediator first and try and work things out on your own anyway!!