What would you say?

My hubby and I have been TTC for almost 2 years. I have a 19yo. from when I was a teen, and he has 2 children from a previous marriage. Our youngest is 11yo. When people find out we want a baby their response is always "Why do you want to start over?" or "You already have 3 kids." We want to have a child together and experience that journey as a married couple. We truly feel it will complete our family. I know I shouldn't have to explain myself to others, but I'm so tired of the comments. This whole process has been emotional and very hard, and dealing with people's negative reactions only makes it worse. What are some good comebacks to say to these people? I don't want to be rude; just politely, but firmly, tell them to f*ck off 😀😀