Should I be okay with this?

This morning my girlfriend, her 20 yr old daughter and myself had a terrible disagreement. I made plans with my girlfriend a week prior to take my Dad to breakfast for his birthday at 9am the following Sunday. We agreed and all was well. This morning I woke and was talking with my partner when she said she was taking her daughter to the gym at 8am. The evening before we discussed that her daughter might go later as she doesn't drive. I was upset and said I thought that we had made plans. My girlfriend said that she had spoke with her daughter and that they decided she would only work out for half an hour instead of 1 hour. I said that I would be leaving for my parents at 8:30. I also said that it wasn't okay they changed things while I was in bed sleeping. She got mad and asked can I wait a few minutes to pick her daughter up. I said no, her daughter is visiting for the summer she is a college student. I have made concessions, she is vegan we buy all her specialty foods. She has never had a job although she is an amazing student. We take her wherever she needs to go. I taught her to drive drive after working all day. She has a regimen and adheres to it like clockwork. The issue is that she doesn't care if it interferes with our plans. I have generally been cool with being flexible. This time I wasn't I set a boundary and left at 8:30 to have breakfast with my Dad and Mom. Now my partner and her daughter are upset and being darr I say beats about this. They also want me to apologize to which I said no. I need some insight please? and thank you!

I agree, you're not wrong in this, you seem to be very flexable and done a lot towards your gfs daughter but not got a lot back. Not a lot of people would be so thoughtful or make the effort if their partner has children from another marriage. Your a giver and Gf daughter is a taker . It's was your dads birthday, they had a weeks notice, you wanted to take him and your mum to breakfast that morning, which is a lovely thing to do. They shouldn't of gone behind your back and discussed other arrangements, not when you were open with them about what you wanted do. I wonder if the daughter put your gf in a difficult position, and knows how to manipulate her, as it sounds like she gets her way a lot. She could of gone to the gym later on that day or if she really wanted to go at that time she could of looked at alternative transport to get herself there. It's a very it's all about ME attitude to have and needs to learn a few lessons in life but that's down to her. I feel your gf was disloyal to you and should of told her no. You don't want this to continue, I would talk to them about this, it's about them learning your willing to be flexible, you want to get along but you're not a push over that is why you made this stand. this whole disagreement as clearly come to a bit of a shock to them!

Hey, I'm near your gfs daughters age, so can give a young persons prospective...although I agree too. Birthdays come once a year and it should be celebrated coz you never know what's going to happen. Your family matter too and it's not all about them. Waiting so close to your dads birthday to spring on you a change in plans, that they decide together alone, wasn't fair. It would make most people mad. Can only think the daughter might feel threatened and be insecure, but you've shown her with actions there's no need to be. Maybe, she needs to see her mum doing things for her, ( she's testing her) like, siding with her when there's a disagreement and gaining her attention, so she still knows she still there for her. Even if your gf is already doing a lot for her, she could still need that reassurance. There could be jealousy if they are close, and if you get to spend more time with your gf then she does coz she's away at college. Talk to your gf alone. Tell her you didn't appreciate what happened, it was for one morning and for a good reason. If the daughter is around whilst you have this discussion and is an manipulator she could make things more difficult/be argumentative. I'd hold back the next time you do something for your gfs daughter. The above advice is right, it's about give and take, so now let her come to you first when she's wants something.

Thank you all for the feedback. :-)