Hi, think Petal02 is right - so much depends on your DH and how well he supports you. Does your DH know your feelings about teh situation, and how does he respond to your concerns?
Also, I agree that there is more to consider here than the needs/wishes of just one child, as you also have to weigh up what is best for your DD, and indeed for the family unit.
You may find that it is a relief to have weekends free - after all, if DSS is going to his mum's for two days every weekend (if I have got that right), then that is a big shift still (with you 5 days instead of 2), but perhaps a silver lining in this is that you will have weekends 'off'.
I have two steps - DSD and DSS - who live with us fulltime and with no 'breaks' (their mother is deceased). I also have a baby DD.
The bad days - I do grieve sometimes for the loss of the idea I had about being a mum (our counsellor helped me to see that was exactly what it was - a loss, and something I needed to grieve myself). I would like sometimes to be able to make decisions based solely on what is best for DD, but I can't, because I am partly responsible now for the other kids' well-being, too. I would like to have more time with DD and DH, but we have two other kids in the house needing help and attention and affection. I've found I have had to compromise on parenting ideas and values, to fit in with the way things have "always been done" with the older kids. So, I won't lie to you, it is tough having two other voices and personalities to cope with 24/7, and it gets a lot tougher when there is a clash of interests! And as anyone here will tell you, it gets very tough indeed when DH plays Disney Dad or 'checks out' of the hard bits of parenting.
On the other hand, there are rewards. I have two new family members, and I get to be involved in their lives in real, substantive ways - which can be very satisfying. I feel proud of the kids and of myself for learning how to do this! And we have good times together, on days out or at meals or just watching telly - and then I think that I wouldn't want to miss out on any of that. The kids are growing up into nicer and stronger individuals all the time, and I get to be a part of that.
I'd still love to have an occasional weekend to myself, though. 