I can't say I feel 'the same' but I hope it is OK if I reply anyway.
I'm wondering what you mean by this - I don't think I 'love' her in a way a step mother should?
I rather doubt that there is any way a step mother should love a stepchild. There are so many different sort of circumstances...
Just reading threads here shows how differently people view that relationship, and what a range of expectations there are. I've read a couple of books about step-parenting that suggest that the level of intimacy to aim for is that of an aunt or uncle. I've heard other people suggest that it is the duty of a stepmother to love her stepkids as her own. I've read posts here by people who suggest that a stepmother can never really love her stepkids and is being intrusive and over-bearing to even try.
Personally, I think love is something that cannot be so easily controlled or created. In my case, with my stepkids, it has grown over time, out of being a family together, and I sort of assume that it will continue to grow. Part of that comes from the kids wanting to be loved, or letting me love them. Part of it comes from the everyday business of being a family, which in my case is maybe more clear-cut because we have the kids all the time. I can't imagine what it would be like if they were only here once a week, but I would hazard a guess that closeness (love, or anything else) would take a lot longer to develop. (I think the age of the kids matters, too. As does the relationship between the kid and their own mother.)
It might be better to ask yourself if you are, and if DSD is, content with the relationship and depth of feeling you have between you, or if you want something more. If you do want more, there probably are ways of cultivating a deeper relationship. If you don't, and if your sense is that DSD is happy, then why worry about an abstract idea of 'should'?