Bothering me..

So I'm facing the fact that my relationship might not work out. I have hope for us but also I'm trying to be realistic that things don't always go as we planned.. And we have 2 kids together. My problem is I can't get over the idea of my boyfriend meeting someone else and then having to share my kids with another woman.. Don't get me wrong, I don't see anything went with step parents, I just dont know how to cope with the idea if another Woman in their lives like that..