Hi Margaret,
Everyone is probably right... stop stressing and it could happen. And you're still young!!
I got married when I was 38 and my son was born just before I turned 40 and my husband was 44. My son is now 6 1/2 (and the love of my life). When we got married, my husband and I decided that we would have 2 children. My husband really wanted 2. I had to be convinced since being a parent freaked me out. Luckily we had no problem getting pregnant. I also work full time so I'm out of the house 11 hours a day. My husband works from home much of the time so he takes our son to school, picks up, does extra curricular things, etc. When I'm home, my full attention goes to my son.
The stress of having a baby and working full time was a lot for me. I have to work so I don't have the option to stay home. I soon realized that there was no way I could ever have a 2nd baby and be a good wife and mother. I then had to convince my husband that I wasn't having the 2nd baby that he really wanted (and still does). I felt extremely guilty that my son wouldn't have a sibling but I also felt that I couldn't give him the attention he needed from me if there was another child in the house. It would all be too stressful for me with another mouth to feed, another person who needed my attention. Plus everyone, including strangers on the street, want to know when I'm having another baby so that made me feel guilty too.
I finally convinced my husband that there was no way I could do it. I have friends who are only children who are happy and have lots of friends and family. I have friends who had their babies in their mid-40s so they are only having one. I make sure that my son is close to his cousins who are his age and make sure that he spends time after school and weekends with his friends from school. I've asked him if he wants a sibling and he always says "no". He likes having my full attention. Having just one child lets us travel easily on an airplane and lets us give our son lots of things that he may not have if we had another child.
My girlfriend has an older brother but he died when we were 14 so now she's an only child. My other girlfriend has a sister but hasn't spoken to her in 10 years. Having a sibling doesn't mean that you won't be alone. Not having a sibling doesn't mean that you will be alone.
So whatever happens your daughter will be fine as long as she has people around her who love her.
Good luck with everything! Hope this helps.