Hi Polpo
I do not think you are being a twat. I think you are perfectly normal and human. Personally, I don't have step children so cannot speak from experience, but I have read about it and know people who do.
I don't think that the heart of the issue is that you are associating DSD with the trauma of the birth. I actually think it goes deeper than that, and that the birth was a brutal reminder of an uncomfortable situation for you.
Human beings are not perfect, and parents can't help but love their own flesh and blood even that tiny little bit more than a step child. What's more, a step child is often a constant reminder of a previous relationship, and is also another woman's child who automatically needs and deserves your partner's attention as much as your own offspring with him do (and you yourself).
The step child is completely innocent and deserving of all the love he/she can get, so it is quite tragic really, that through no fault of his/her own, he/she may cause deep feelings of resentment or discomfort to a step parents. You have to feel sorry for the step child because even if they are oblivious to the fact that step parents rarely love them like their own, they are going to pick up on the feelings and suffer for them in some form.
I think you were looking forward to the birth of your child for all the usual reasons, but one of those reasons may have been that finally you would have a child of your own who also really needs your partner, just as much as his step child.
I am sure you have been frustrated because the fact that your husband's ex has a child with him gives her some power over the way you conduct your life with your husband.
It would have been horrible to then finally go for the birth of your own child, and be traumatised by the very person who will always be there and need your husband and compete with your own biological family's needs, in such a physically intrusive and traumatic way.
It stems from the primitive side we all have, and it is not right, but I wouldn't judge you negatively if that is how you feel. You are only human! But anyway, I thought I'd take the time to write this out, because I feel for you. I think you need to factor in that it isn't just this incident that has upset you, and I don't think you are mad at your darling partner so much as at the finality of the situation and the 'unfairness' of not being able to get the level of control and privacy you want over your own life with your own little family.
I think if you go for counselling, it should be with someone who specialises in stepfamily dynamics. Maybe you need to develop internal ways to get some mental space from your husband's ex wife and child, whilst still being a loving step mother towards the step child who is blameless in the situation.