I think you are getting a hard time here OP, probably from lots of people who have never been in your situation. Even people who have commented as step parents aren’t in your individual situation.
I understand that children come first but it’s human nature to be uncomfortable with any big changes - positive or negative. You had a settled routine and circumstances have changed. It’s one thing to be aware that a hypothetical situation might happen but there is nothing wrong with struggling with it when it does.
Both you and your daughter have had your lives turned upside down and now have to make sacrifices. It’s clear you care deeply about your step daughter but there is nothing wrong with being honest and saying you are struggling, I think most people would be in this situation but wouldn’t admit it from fear of judgement, based on there responses it’s clear to see why.
You have this extra responsibility now and I think it’s admirable you say you have chosen to take on a motherhood roll without expectation, you have essentially gone from having one child to two which is a big adjustment for anyone.
I might get flamed for this but as much as I think it’s possible (and essential) to treat a step child the same as your own, I don’t believe you can ever love them the same.
In most circumstances you weren’t there for the beginning of their lives, you didn’t see them grow up or contribute anything to their development, if you split up with your husband it’s possible you won’t be in their life anymore. This means you split your time with your own child for a child who you don’t have the same bond with.
Most people take the same attitude that you knew what you were getting into when you married someone with a child, the fact is you didn’t! No one can possibly know how this would play out unless they are in that situation. Maybe you should post on the step parenting board where you will be judged less harshly.
I think when it comes to step parenting it’s the only situation in life where you are told your feelings don’t matter and the children come first and you should just put up with anything or leave. I believe life isn’t as black and white and it’s it’s obvious there will be struggles but being honest about these feelings means you are more likely to fix things. I don’t think you should just leave, can you not talk to your husband and look at ways to improve things? Maybe if he just acknowledges it’s hard for you then you might feel a bit more validated and like you aren’t just a robot who has to accept anything without complaint.
You have had some good advice and sympathetic responses from people who haven’t just answered without thinking how they might feel in this situation. I think the “you knew what you are getting into comment” is so unhelpful and lacks so much empathy.
Does your stepdaughter have any other family she can spend time with? Can you plan date nights with your husband and get a baby sitter for both children? Can you spend time with your own children separately so they are both getting one to one time with their parents?
A pp suggested family counselling, that might be a good option.
As much as you feel like leaving I don’t think you want to deep down, you want the fact you are struggling acknowledging and to not just have to put up with everything with no consideration to your own feelings. The only way that can be achieved is by being honest and talking and maybe making changes to improve things.
You sound like a deeply empathetic person who wants to do the right thing but also wants their own feelings considered. Ignore anyone who has tried to make you feel like a wicked stepmother.
I really hope things improve for you, it might just be in time that you will get used to the situation more and find it easier to split your time. You might have to put more effort into making it work. Sadly you might find it doesn’t work at all and choose to leave. Whatever the scenario there is absolutely nothing wrong with feeling the way you do. You say you are treating both kids equally and that’s all you can do, it’s impossible to change how you feel inside and it’s wrong to judge you on it.