OP there is no rush, you are only 35. I would try and access some counselling to work through your thoughts.
You have 3 children, the youngest is 9 so I'm assuming the other two are at secondary school. How would they feel about another baby? Are you not incredibly busy with the 3 you have? Are they not times now where you need to be in 3 places at once? What about the supporting your teens through their gcses?
Importantly, why had you not thought about having another child before now? You must have run this over after the 9yo was born. If you deceded you were done then, why?
Logistics, have you the room for another child, the 9yo can't share with a baby. Do you have bigger enough cars? What about finances? can you afford another maternity leave or break from work. Can you afford another child?
You are nearly at the end of kids at primary school. Do you really want to have to start back at that in 5 years, and another 7 years of primary?
What about you and your career? You have children who can look after themselves a bit more and help around the house, are you not looking to focus on you and your wants and needs? Now is a time you can find yourself again and do hobbies, sports, go out in the evening without a babysitter. In a few years your be able to holiday without the kids and take advantage of a cheap week in June somewhere hot.
Do you think this is a fear of empty nest, or not being needed as much? A fear of what's next for you?