How did you know you were ready for another baby?

Just wondering how and when you knew you were ready for another baby? My LO is 15 months old and my husband and I arent getting any younger and we want another one but at times the thought of another baby scares me to death. Sometimes I feel like I cant do right by the one I have now. I definitely want her to be a bit older because I don't want her being so dependent on me since a baby will be. I also just try to think about daily life and my routine and how it will go with 2. I'm the one who takes my daughter to the sitter so I'm the one who has to get her up and dressed and ready and usually our mornings run smoothly but then I think of adding one to that. I'm a teacher so I stay stressed out anyway. Also, the first semester went well this yr but right around the time my LO turned one it was like she stayed sick and I always stressed in the mornings because I was afraid she would have a fever which she has had many times and then I have to take off work and that just adds more stress because I have to have sub plans. Like I said, I know I'm not ready for another one just yet but when do you know or think you know I should say.