I think, like others, you need to go and get some form of counselling to help you with YOUR loss of not being able to have another child.
Because it really isn't healthy for you to be living like this, feeling so anxious and upset about not being able to provide a sibling for your child.
FWIW I believe that no one should ever have another child in order to provide a sibling. You should only ever think about having another child if it is for your own sake. There is no guarantee siblings will get on, will play together or will even like each other - either as children or as adults.
I have a 12y DD. She is an only child. Yes, there are times she would have liked a sibling. Just like there are times her friends with siblings would prefer to be an only child!
DD knows that we would have liked another child but it wasn't possibly. She doesn't hold that against us or feel badly about it. Its just one of those things.
But DD is happy, really happy. She is well adjusted, she is surrounded by family who love her and she is so sociable - always surrounded friends, either in person or interacting electronically. She doesn't have these so-called "only child traits" - she happily shares, takes turns, enjoys copany, also enjoys own company, etc.
Being an only child is NOT a bad thing. It is just different to being a child with siblings - neither is always the best situation. They both have prs and cons, and you have no way of knowing if your child would have even got on with a sibling anyway.