Emotional support.. 😩

So my boyfriend has a kid with another girl, I love him and the kid but I don't want a baby with my bf.. It would be my first and not his and it emotionally kills me knowing I can't share that first child with him.. He wants more kids but it just eats at me, I'm afraid he won't love our kids as much and I'm jelous and I can't help but not be. I know it's selfish I just don't know how to over come it.. 😔