Help. Advice on Kids.

So all my life I've known I don't personally want kids. Or that has always been my belief. I'm not good with kids. They make me not me. I'm also now in a relationship, going on 4 years, and he has 2 kids. They however are living with his parents since before we got together. They do however visit often, but I'm not mom. They have one, however she's another story. She wants to be mom and I feel like she's fought hard to become who she is and she deserves to be able to be around. Now the advice I want to ask for is;

If I never wanted kids, why do I get so jealous and mad that he has kids? Why am I up late with the thought that I got cheated out of starting a family with someone I am with? Why do I occasionally imagine having maybe just one with him? Why do I even want to have one? Do I even want one? Can I even handle it? Is this a normal thing? Should I wait to even see if this is a continuous feeling or should I just forget about wanting a kid and shut that thought down? I'm so lost. I mostly needed a rant to people who may have some outside advice that can help.