Two things jumped out at me:
One - I see so much on here saying that blending doesn't work and isn't fair on the kids etc. on the one hand it makes me feel selfish. But then don't we all deserve love.
I see your response on here so much and I think you are both right and wrong. Yes, we do all deserve love and to be happy, but you have still made a selfish decision. You do not have to be married, co-habit and blend families to have love and be happy. You could date while maintaining two homes - either permanently or until the children leave home. You have chosen to bring children into the world and are responsible for their health (including their mental health) and wellbeing/happiness. You cannot make romantic decisions in the same way you can when you are young, footloose and fancy free. It is hard, and I know as I am in this position, but a right to happiness doesn't mean a right to be selfish when you have children.
Two - My daughter adored my dh at the start. She wanted him to move in. She disowned her own dad and wanted to change her surname and all sorts. Sadly her mental health went downhill a lot a year later. She has asd and adhd. She witnessed her own dad being emotionally abusive to me and always shouting etc.
It sounds to me like your daughter had seen things no child should have to see (which I am not blaming you for). She probably did adore your current DH but on a very superficial level at the beginning. He will have been nice and making a big effort in the early days, in her eyes rescuing you from her abusive father.
She probably did like mummy's boyfriend, but she was reacting as a juvenile would, in very simplistic terms without really understanding the reality of the situation, which is where the adults should know better and step in. She wouldn't be able to appreciate why she was having the feelings she was having and what you getting married and moving in with him would mean for all of the children involved. She has to share you, share her home, deal with another child in the mix all while dealing with her own issues.
I am not trying to pile on and make you feel bad about your choices and I do not mean to sound harsh. I think my points are that you have to make your decisions with your children's best interests at heart (which may mean sacrifices on your part) and that you have to consider what is behind your children's reactions/emotions and not always take them at face value.
Your situation now has moved on and with the benefit of hindsight, perhaps you would now chose to wait for marriage/cohabitation if you went back in time. But you have to find a way to move forwards from the situation you are in now. That may be to try to separate your lives a little and spending more time in each family unit rather than trying to force a fully blended situation, even though you are living in the same house and married. That may be easier in terms of removing your little family from your step child as he is not there are much, but maybe give some thought to how involved your DH should be with your children - e.g. should he join holidays or do school runs etc, what about joint family days out (should they be knocked on the head for a bit or maybe reduced in frequency) ?
The best I can suggest it talk to your DH and try to agree some rules for how to move forwards to try and meet the needs of all the children. Good luck.