I could ditto almost everything you've said. I'm new (to posting) here too but posted a couple of weeks ago and got lots of support from more experienced step-mums.
I too get that sick feeling that you describe and think that it's natural - mostly because I don't actually want another woman controlling any aspect of my life, although I understand that when her children are involved she might want to do this. There's lots of good advice here and the thing I read often is detach.. detach... detach... I've been trying to do that every time I get the sick feeling.
A couple of months ago I read a post I liked so much that I saved it (I don't actually have the link to the post - just the words) so apologies to the original poster for not giving them credit. I hope this helps you (it did/does me).
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Lord, help me to be a good stepmother? give me the patience of a saint, the strength of an ox, and the skin of a rhinoceros.
Help me to understand why their mother would rather believe her children are suffering at the hands of an evil stepmother than being loved by a kind, responsible adult who brings them happiness. Let me be content in my knowledge that children have an unlimited supply of love to give. Keep me from reminding their mother that my presence in their lives does not take away the love they feel for her as she fails to understand that a child?s love multiplies and does not divide.
Help me in the constant struggle to find my role in the children?s lives, somewhere ?below? a parent but ?above? an aunt or family friend. Accordingly, help me to appropriately and accurately represent our relationship when in public. If someone calls them my children or me their mother, guide me in how I am to respond? let me know if I am overstepping my bounds by remaining silent or disowning them by correcting the assumption.
Help me keep my patience, even while dropping my plans at a moment?s notice when their ?real? parents are busy? and when their mother tells my husband ?she is not their mother!? and ?why can?t she watch them?? in the same conversation, give me the strength to keep from going over the brink of hysteria.
Help me to know when and how I should discipline the children without exceeding my authority or taking too passive a role. Lead me from the temptation of spoiling them in the vain hope that they will accept me and not resent my presence. If a time comes when I disagree with how they are being raised, make me keep silent and not object in spite of the fact that they live in my home.
Help me to have the premonitions of a psychic and the perception of a mind reader. Give me the ability to anticipate all potential problems and to accept the fact that they will usually be my fault. Make sure I am always ready with some cash in my pocket for surprise expenses and the time to drive somebody somewhere at the drop of a hat. Remind me that if I have a moment to catch my breath, I have probably forgotten something, and when I finally remember what it is, do not let me assume one of the ?real? parents is taking care of it.
Help me to forget that in spite of devoting the time, money, stress and energy required in raising my stepchildren, I will not shop for prom dresses, be the mother of the bride or dance with the groom. Remind me that maybe, some day, the children will give me a second thought and remember something positive about me from their childhoods. Let them gain something from having known me, no matter how small or insignificant, and let something, anything I did make them each a better person, whether they realize it or not.