I have been with my DH since my DSS was 14 months, now near 14. We went on to have a DS who is now 10. He had him full time so I did know that this my situation from day 1 but as a childless woman I don't think anything could have prepared me what being a full time step mum would be like. I love him with all my heart and he is my son. I don't refer to him as my SS, just my son. I am mummy. However it has not been an easy ride but that is due to him being autistic, growing up and taking his anger out on me. I would be bruised, black eyes ect. but noone else used to get the violent outbursts. Thankfully that all stopped around the age of 8 but I did start to consider at that age could I continue. What if the violence continued into his teens when he was bigger and stronger than me. I was resentful. I used to cry that this was my life but I also considered how would I feel if it was my biological child and I thought I would feel the same!
We used to have a break from both kids as my inlaws used to take them a few weekends a month but we are no contact with them now.
We do things with each child separately, luckily my family accept my SS as our own so they will have one while we do something with the other and that way we get to give each child dedicated time, although only every few months.
Now my teen is not coming out with us as often I actually feel incredibly guilty that he thinks I don't want him, although I know that's irrational as he is just a normal teen.
You can look into counselling for you if you are happy to stay in this relationship as you don't want her to pick up how you are feeling as she will feel abandoned. I grew up with an absent father and my mums boyfriends would come along, be lovely...split up and never to be seen again! It takes a toll. She may benefit from this too if she is going through abandonment issues. This may help the family as a whole.
I feel 6-9 was the biggest struggle for us. He is now 13 and we are very close. He is affectionate, funny, kind and caring. I love him with all my heart and I can go for weeks without not even thinking about him being a 'step' child. I love him just as much as my biological one and I say that hand on heart.
Sorry, I know there is no real advice there but it is either you accept it and accept her 100% or you leave now before she gets more attached and remember more as she is older.