Feeling overwhelmed

So I'm pregnant with my first baby. As time for her to arrive gets closer and closer, around 50 days left, I'm feeling beyond overwhelmed. Honestly I never seen myself as a mother to begin with but I've been beyond excited this entire pregnancy. Here's the issue now, I also have a step daughter and step son, I never pictured myself with one baby let alone 3! I love all 3 of the kids more than life itself but now I'm going crazy. My step kids are going to be coming around more and having a newborn trying to adjust to being a new parent I just already feel myself freaking out and feeling like I'm going to fail as a parent.

My step kids haven't been around since me and my husband got engaged because of their mother so I've never had any children in my home. He's getting court order visitation so I go from taking care of no kids to 3. I just don't want it to put a big strain on mine and my husbands relationship.