To have or not to have

Melissa
Maybe this is a silly question but I need some advice. I'm almost 29 years old and I just had my first baby a couple months ago. My husband is 42 and we also have his two older children from his first marriage full time. His children are 11 and 9. Our life is full but my heart is breaking at the same time. I really think I would want to have one more child in a few years when my baby starts preschool. I want him to have a sibling close to his age. The whole time I was pregnant I felt like this was going to be my only child but now that he is here, I am just so filled with love and joy and he is growing up so fast. I feel sad that all these exciting things I'm getting to experience might be the only time I get to do this. How do you decide you want another baby or whether or not you are done having children? My husband and I are both active and healthy if the matters as far as the age thing goes. Please advise.