Howwwwwwww do you get over not being first?

Lately me and my boyfriend have been talking about marriage and kids of our own. He has two kids from a prior marriage and while I do want all of these things with him, every single time it’s brought up, it stings knowing that I’m not the first wife or my child won’t be his first or even second child. I struggle so bad with wanting to know that my child will be loved and cared for just as much as the first two by him and his family. And while he always reassures me that it will be, it still stings every time to think about. And every time basic parenting stuff is brought up, like the kids school stuff etc. that him and HCBM have to discuss, it stings even more knowing that he has that connection with another woman and not me. Howwwwww do I get over this?? I know it’s all my own issues and insecurities and in my own head. But how do I fix it?? It’s breaking me