Baby #2 and scared

I am the mother of 1, from a previous relationship. It was mentally abusive and unhappy more times than not. I am with someone now who treats me wonderfully and loves my son to the moon. But after finding out I'm pregnant with my second, his first, I'm terrified. I am feeling guilt and regret. I know I will love the baby, but it still seems unreal. I don't feel as excited this time around, which isn't fair to my boyfriend. Has anyone felt this? Has anyone felt bad about having two babies with two different people?