It might help you to understand that even in non-step families, the demands and time you have with your partner/various children never remain constant. I have one child in a non-step family. I didn’t have a lot of role models for how other families operated at the time I had a baby (no close friends or family with young kids) so I didn’t “know what I was getting into” as the pet phrase here goes. When my kid was a baby I had reasonable 1:1 time to spend with my husband. When she was in elementary school we suddenly had less time together due to the changing demands of parenting, her need for different types of attention from us, and her activities. That got tougher and tougher as she got older. Now that she has gotten into high school and her activities are more independent in nature, we have more 1:1 time together again. And with a different kid who liked different things or needed different kinds of attention that pattern might be different. Family life is unpredictable for everyone - you should never assume that things will stay the way they are or that you are guaranteed a particular amount of priority or access. In any family with kid(s), you can pretty much assume however things are this year, it’s going to be different next year!
You have asked for advice on how to deal with the resentment - as much as you might yearn to recreate the family environment you had before your stepdaughter came to live with you full time (seeing that environment as “what you signed up for” and the stepdaughter as ruining things/stealing time away from your real relationships and family), please try your hardest to not create a two-tier family. Don’t ship her away frequently to relatives under the pretense that she needs to see her mother’s side, simply so you can restore what you think you are entitled to. She needs to see them of course but don’t use this as a convenience to remove her. Don’t leave only her regularly with a sitter and then take yourself, baby, and DH off somewhere fun. As much as you see her as an intruder, your DH has been her father for longer than he has been your husband. Kids, especially ones who are in vulnerable situations, are remarkably perceptive at picking up when they are welcomed/wanted or not, much more so than adults think. It’s a key survival strategy. Surely you can think of colleagues or neighbors who give you a cold “they don’t quite like me” vibe without doing anything specifically rude to you…anyone who thinks kids are less perceptive than adults is fooling themselves.
In a household with two kids, one on one time with ANYONE is precious, whether that be time with your partner or with a specific child. Try to organize those 1:1 times for everyone with everyone else, (eg 1:1 SD:dad time, and time for you and partner alone without both kids, as well as occasions when SD may be away with relatives)… and not in a way that singles out the stepdaughter to be removed more often. If you keep on viewing the group of you, your child, and DH as “the way it should be”, it’s going to be really hard for you to get over the resentment. Accept the new situation and work to create 1:1 times or all permutations of sub-group times for everyone, like in any family with two kids.