I am not an older M. myself, but I have a few close friends and family who have been in your position.
My aunt and uncle had 2 children, 1 a jr in hs, the other a 7th grader. They were 100% certain they were done and believed my aunt had begun menopause when they found out that baby #3 was on the way. They had enjoyed almost a decade of having the freedom to go out, take expensive and exotic trips, etc and were not looking forward to giving this up for the new arrival. This child, Dan, just started hs this year and my aunt and uncle keep telling me what a blessing he has been to all of them. They credit him with helping them to grow in ways they never expected, as the stress of caring for a baby was certainly more apparent to older parents.
My best friend in hs was the youngest of 3 kids when, during her sophomore year, her "oops" little brother arrived. She was very resentful for about a year, but as her older brother and sister went away to college, she and her baby brother grew extremely close and she grew so much in her loving and sacrificing for her brother. A few years ago her older brother was tragically killed while serving in the military, and the younger son has been a tremendous source of consolation for the whole family.
I am sure you are in shock right now. Truth be told, if I were to get pregnant at this very moment I would also feel sad, as I do not feel ready for a new baby right now. I would allow my self to be sad for a while, and then dive head into making sure I had a healthy pregnancy and was preparing to welcome my baby as much as possible. I would also try to use the pregnancy as a time to do as many adventuresome things as possible before the baby arrives, since it will be a few years before you again have that liberty. It might also help for you to put on a super happy face for your daughter. The joy on my older son's face really helped me be happy for baby #2 (who I did not feel ready for). How could I feel so exhausted and sorry for myself when my first son would just beam every time someone would mention the coming arrival?.