There seems to have been a lot going on in very little time to recompose your family, it's no surprise that there is little control over what is going on.
Yes, you might be able to make it work, but you need to slow down. It takes time to make a relationship work, takes even more time to get step-children to accept you, and still more to get the whole wheel turning without too much trouble.
I am sincerely sorry for your miscarriage, having experienced it, I know how emotionally devastating it is, but please do consider whether being pregnant at this stage is the right thing to do if you are thinking about trying again. The situation is: OH starting a new business, stressed and overwhelmed. A relationship that seems to have more ups and downs. Two children who are trying to adjust to someone else coming into their lives half of their time, a toddler to give time and attention to, with a dad not in the picture, emotional baggage on your part.
It really really isn't the time to bring yet another child into this. Take a step back, concentrate on your relationship and step-children and don't expect too much. The priority is to get everyone to feel loved and valued, the rest really comes next.