When people show you who they are, believe them.
-Maya Angelou
Look at this situation and believe it for what it is. It is not going to magically change without major effort… none of which is forthcoming.
They’re probably going to get worse. Even kids who aren’t used to running the show can grow into unbearable teens. Think about the sorts of things teens want and imagine what will happen if she continues to get her way at that age. It won’t be any easier if your girlfriend suddenly decides to put her foot down after allowing this to go on for seven or eight more years.
Honestly sounds like there are problems on both sides here. Kids can be difficult, if you’re expecting a perfectly behaved little angel you’re not being realistic: they’re their own person and by nature immature people who like to explore and test boundaries. You don’t think her behaviour is normal, but actually throwing tantrums, laziness, bad manners, etc is fairly normal for kids, it’s more about how that behaviour is managed. Equally though it does sound that your GF and her parents do have issues with setting appropriate boundaries for her child and sticking to them.
However to me the crux of the issue here is that your GF totally reasonably puts her kid first, whereas you by your own admission don’t like the kid. That seems like a big problem to me. However if you are really committed to the relationship it seems to me the most you can do is to change your own behaviour and expectations as changing the behaviour of your GF, her child and her parents is a fairly big ask and is not something that is just going to happen because you want it to happen. Of course that’s not to say you can’t do anything or cannot expect any accommodations, but just don’t expect the mountain to come to you.
We just broke up. Thanks, everyone, for your replies.
Sorry to hear that. I hope the future has good things in it for you.
You must be feeling a lot of sadness amidst the relief of making a decision. I’m sorry for the sadness and wish you happier times soon.
Appreciate the sentiment; thank you. It was actually her decision. Still a relief, because I know my indecision was tearing her up inside, and I felt awful about that every minute of every day. She put up with it far longer than I deserved.
This is something that single mother’s* need to get a grip on. This idea that the child ALWAYS, NO MATTER WHAT, comes first.
Nobody should always come first. Sometimes it’s the child, sometimes it’s the mother, and sometimes it’s the BF. But nobody should always come first. It’s not fair to the other’s involved.
*Why yes, you may switch out fathers for mothers if you would like.
I’m sorry you are going through this.
Kids are hard. A recentish study shows that having a kid causes the same drop in happiness as losing a job or…losing a spouse! And a lot of that “happiness” hit is directly because of the way children test and transform a relationship.
A kid isn’t going to fit seamlessly into the adult life and relationship you seemed to have wished for. It’s just not how they work.
The kid’s a ginger, isn’t she?
I don’t know when the “boy friend” (or “girl friend”) should go first.
As a non-parent, I’m not sure that this kid is being raised correctly. But the person “dating” a single parent doesn’t get much more of a vote than I do. They can commit to greater involvement or vote with their feet.
If only you had used some other identifier than “and”!!!
Had to go there, didn’t ya! 
I mean like, c’mon single moms! I know that little person depends on you for love, food, shelter, and all of the other basics for life and a healthy future, but I need a blowjob NOW.
ReticulatingSplines, I’m sorry for your trouble and hope you are okay.
I get your snark but he actually has a point. Your kids can be first but making the explicit statement “My kids come first period!” to potential SO’s can drive off decent and reasonable people. I made very similar statements when I was dating 15+ years ago and in retrospect I can see how it would give someone pause in dealing with you if that’s the attitude you are espousing. It feels great ego-wise to say it but being on the receiving end is another matter.
Cooperation is fine but if you are being told you are (effectively) a second class citizen from the start there are often better relationship options out there.
You and your girlfriend have different parenting styles. Those behaviors aren’t wrong, they just are ones you wouldn’t want to see in your own child. But this is her child. If those behaviors will create conflict for you, then you are done and need to get out. And don’t sugar coat it - you will have to put up with that parenting style until your girlfriend’s daughter leaves the house - which might be 18 - or might be 28. That is a LONG time to deal.
If you want to meet a good single mom who has your same parenting values, go to a “parenting class”. Ask at a local school where these are held - usually when school is in session.
Anyway plenty of single moms there and they could use a bit of help from someone like you. They will agree with your rules!
I just spoke to my friend, who had a “my daughter comes first” policy while he was between wives, about this. He remains strident on the point: “Until the child reaches a certain age, they always have to come first. Period. One hundred percent of the time.” I asked him what he considered to be “a certain age,” and his reply was, “When the child is old enough to do for themselves.”
I don’t know if I got the sense from the OP that he actually set out to meet a single mom.
Plus, trolling for single moms. . . eeeeew.