feeling like a shity mom

My whole life, since knowing I could be a mom one day, I have dreamed of having kids. I have always been the nurturing type. I have taken care of so many other people's kids over my lifetime. I was a nanny for almost 6 years. I've always thought I'd be great at this...motherhood. My son is 5 weeks today and I feel completely lost. I've cried so many times because I don't know what I'm doing. I thought I'd be so much better at this. I really thought it would be easier than this. And now I feel like I've let my husband down because I used to say how great is be at this. I love my son with all my heart and it hurts that I can't be better. I'm so heartbroken right now. It doesn't help that I homeschool my 9 year old step daughter who is diagnosed with bipolar disorder. I'm so worn out. I feel so guilty when I want to just run to the store alone. Ugh I don't know what to do!!