Some replies have gone veering towards the crazy I see!
OP, my advice is to reframe this in your mind.
Read a few books on only children. Think about why you really want two and why one is ok (there are plenty of reasons) . There's a couple of Facebook groups too.
2 looks like hard hard work.(Another reason why I didn't have a 2nd in the end was because my daughter was never an easy child oh and her dad is lazy and I was not prepared to do all the work raising 2).
Make a list of what you can do with only one child. You will certainly save money!
Plan a holiday? Go somewhere fun you have always wanted to-maybe by train. ? 7 year olds are great company.
Do you want a play mate for your daughter? Be the playmate! I see mums of 2 kids won't get up to play with their kids as they think they shouldn't have to (no offence or mass generalisation meant).
I didn't want my DD to feel lonely so I decided to be a big kid sometimes. Do dances in the kitchen, put a face mask on and watch a film. Find an active hobby you both like, or a crafty hobby.
Even do jigsaws and colouring together. Run around outside.
I am really not trying to be flippant or unkind.
But I do think you need something to distract you to shift your focus.
Switch it round. Your house need not be full of kids to be filled with love. It can still be fun.
Another thing you can do which I have found helpful is that you can be a positive presence in kids lives without being their parent. Why not get to know your daughters friends a bit more?
I try to chat to my DDs friends and it can be lovely. They are funny.
Push them on the swings while it's summer, or invite them to the cinema? You'll have company of more than 1 child and they'll enjoy it.
My DD likes going to friends houses as there's more kids there but it works both ways; the ones of 3 or 2 like coming to our house too as it's a bit quieter and they can play in the bedroom or garden (free of younger siblings!).
I don't mean any offence by this and I'm not trying to minimise your heartache or be patronising. But I'm sure your daughter would appreciate a change of mindset while you figure out what to do x