I realise you might not be coming back to this but I've found this thread fascinating and it's lovely to see someone being so open, honest and insightful. And so many people have said such useful things. The most true being that, really, no need to worry about there not being enough love to go round.
I wonder if you're being a bit hard on yourself, imagining you'd be jealous, when most people aren't and you seem well-adjusted. I get the impression that you've worked hard to become who you are, and maybe you don't quite trust yourself to hold up under such pressure and change, but the depth of thought over this suggests you will.
I had a bad childhood and for a long time my main reason for deciding not to have a child was that I didn't think mothers just naturally loved their children and I couldn't put my child through that. I just assumed I wouldn't love my child and so would be a terrible mother.
A few years of psychotherapy later... I can't believe my stupid childhood almost robbed me of having my beautiful little girl! And it was having her that pushed me to realise my dreams in terms of my career and home. And while the first years nearly broke my relationship, that was really due to pre-existing issues and we are now much stronger than I think we could have been without this experience.
It changes your life completely, and many relationships don't survive. But if you feel your relationship is strong enough that the practical aspects of raising a child won't create serious friction, then the only issue really is whether you want one, because you really don't need to worry about rationing out love or, I believe, jealousy. Having a child is inexplicably hard, but also inexplicably wonderful. You can't say this to people irl, but I'll say it here: I think people who are emotionally capable of being parents but decide not to have kids are missing out. Children teach you so much more than you could ever teach them, and there's no way to explain the love.
It sounds like you're a very cautious person who weighs up all the risks, but perhaps not the benefits. As people have said, you want to be with your partner for your whole life. How do you want that to look over the coming decades?
You have time, but I have to say my biggest regret is not having my daughter earlier (I was 34). Admittedly I wasn't emotionally equipped any younger so it wouldn't have been a good idea, but I never imagined I'd want a second and now I'm creeping towards 40 and just starting to try. My body is against me!
The people who have said that the depth of consideration you're going into over this suggests you'll be a good parent are 100% right. You'll be a conscious parent, and you and your partner will work together to make good decisions for your child.
And everyone is right: if you don't want a child, that is absolutely fine. You will have a much easier and more luxurious life! (Apart perhaps from those final years. ☹️)
My only pause in this whole thing, and a few others have touched on it, is whether your caution is partially because there are hidden issues in your relationship - that you feel you can only maintain this level of happiness in a bubble. I think and hope this isn't true.
It does seem that whatever you choose, you will be happy, because you have such a solid relationship.
Good luck! Very much agree with everyone suggesting therapy.