There are so many positives to being a mum of a single child, maybe look at those?
Time- for yourselves, for your DD, as a couple, family more likely to cope with babysitting one child
Money- less childcare, less food bills, easier to work full time, can afford to give child more experiences, more holidays, can save more and don’t need to work as long- can afford to get them bike/ phone/ help with uni etc
Energy- more energised having less cleaning/ washing/ care etc and not devoting energy to more IVF, less chance of dealing with fighting children, more peaceful home
Age appropriate- can do things without younger or older siblings to hold back
Space- no need for bigger house, can have a small car, less clutter
Health- less birth injuries, pnd, breastfeeding pain, pregnancy weight, better peace of mind and less stressed, avoiding further ivf injections, procedures, medications etc
Connection- hopefully close relationship with child and less strained marriage
Age- chance of abnormalities increased with age, miscarriage would be painful, means you get to spend more childfree time at the other end, won’t be very old parents
Portability- easier to travel with one child
Environment- much lower impact and easier to focus on environmentally friendly choices