Hi,
I have been in an almost identical situation for just over 5 years. I have one stepchild, and met my partner when he had been separated from the mother for 7 years, so most definitely was not the Other Woman.
It has been one of the most difficult things I have ever had to deal with. Really, I think that people who have not experienced the harassment of an ex that can't move on have no idea of how bad it can be- so first off, I know how you're feeling!
Our situation is still not perfect (I'm writing this at 5am, unable to sleep because I'm stressing about the latest nonsense to do with summer holiday plans!!!) but our situation has improved greatly due to my fiance and I setting some very firm boundaries.
Firstly, accept that she will never change. All you can do is change the way you react to her. I can't stress how important this is.
Secondly, insist that he goes to mediation with her and that they work towards a clear,all-encompassing, mutually agreed, written parenting plan. This will be hugely beneficial to the child and the adults involved (including you!) will have a very clear sense of their respective responsibilities and boundaries.
After 2 1/2 years of putting up with constant arguing, criticism, late-night phone calls, changing of contact visits at the last minute, demands for more money etc etc, I found a family mediation service, which my fiance attended with his ex.(She, of course, didn't know that I had anything to do with it!)
While, again, the results weren't perfect it had a hugely positive effect on my life and, most importantly, that of my stepchild. We now have a structured parenting plan that, for the most part works. And the effect on my day-to-day life has been positive. For example, the mediator recognised that the parents were unable to talk on the phone without an argument ensuing, which my stepchild would often overhear, so now they communicate by email, using the phone only in emergencies. This has meant that i no longer have the intrusion of repeated phone calls at any time of day or night, and having to listen to my partner deal with them in my home. My life is much more peaceful!
Which brings me on to my third, and most important point. Take a step back and protect yourself.
As much as you love your partner and stepchild, the conflict between him and his ex is his past, his baggage, his to deal with.
Of course you want to be supportive of the person you love! But many men are so relieved to have someone "on their side" after years of dealing with a difficult person that they can over burden their new partner. That was certainly true in my case.
He doesn't have to tell you everything.
For example, do you really need to know about the critical, unpleasant and untrue things she says about you? Do you really need to know that she thinks his child hates you?!
I now have a rule that I only want to be informed about things that directly concern me- ie dates of visits, changes in the financial situation, my stepchild's welfare while staying in my home.
For example, when I recently got engaged (which my stepchild was really pleased about, by the way) I knew that the ex would have something unpleasant to say about it, but I told my partner that I never, ever want to hear any of her opinions either on my engagement or our wedding. Why should I let such a wonderful, positive event be tainted by someone else's unhappiness and jealousy? What she thinks about it is irrelevant so I choose not to hear it.
While you are still embroiled in his conflict with this woman you are both stuck in the past- his past. You must look towards your future together. If you know that you have a good relationship, that he is a good father and that the child has a happy time in your home that is all that matters. What she thinks about Christmas presents is just so much hot air!
Well,I feel better now anyway- writing here is quite therapeutic, isn't it? I hope this is helpful in some way.
I'm going back to bed!
Clea