I am 44, and my DH is 49.
We met and married 3 years ago.
We both have children from our previous marriage (I know this part is different for you), and we both love and care for our blended family. Everyone is treated equally.
When we met, we both said we didn't want any more children, we were content with our existing children, and we were so happy we'd found one another, that was our miracle. Our ages also factored into this. We wanted to travel, keep excelling professionally, and support our existing children, all of which we have done and we continue to do.
However, I am currently pregnant (natural conception and planned) with our first child together. I'll be 45, and my DH will be 50 when I give birth.
It is a completely, completely different feeling to having our own separate children. We love our existing children but this is our joint child, 50% him and 50% me. It's hard to explain, but we feel this is part of our bond with one another and enhances what we already have. He's also loving the experience for the stage in life that he is at, me not so much with the nausea! And, it may all go wrong, and we may not be successful in the end, but I'm so glad we've tried. We'd have regretted it otherwise.
I cannot imagine not experiencing my own biological children (health permitting), especially making an active decision not to have any of my own at all due to the partner I've chosen.
Your partner has her children, and while you're part of their life, it is different to having your 'own'. And you're right. If you split up, all you'll be is a memory. It's fine that your partner doesn't want anymore, but it's also fine that you realise you'd love a child of your own.
I understand you both talked about this, but you are allowed to change your mind, I did, and you are allowed to pursue your dreams.
You live once in this life. Should you decide to, it will be very hard to make that change, but it'll be even harder if you don't.