Am I being a brat?

I have been with my partner since highschool, we have 2 kids together (4&5 years old) I want one more and he doesn’t. I know this is common with couples but not sure what to do. We are financially stable, have room, etc. He says he just doesn’t want to start over.
What I don’t understand is we already had another baby on the way, twice actually. I’ve had 2 miscarriages since our youngest and now a year later, when I want to try again, he has decided he doesn’t want anymore. It makes me feel like I went through the miscarriages for nothing. Like why get me pregnant if you didn’t want another one? I am getting older and don’t want to wait around ya know? I love him but am seriously considering leaving since we can’t agree on it. I am firm in wanting my rainbow baby but don’t want him to agree to appease me and end up resenting me or the baby in the long run so I stopped mentioning it. Just the conversation has put strain on our relationship.