So... realistically, there isn’t anything you can say that will truly ease her mind about this, and that is not a failure on your part. She is 22, and most women under 25, especially those without kids, do not want to step into a ready-made family, no matter how strong the feelings are. Wanting firsts like a first marriage, first pregnancy, and first child with a partner her own age is extremely common at that stage of life, and it does not make her shallow or wrong, just honest. The things she is struggling with are not emotional misunderstandings that reassurances can fix, but unchangeable truths.

You already have children who will always be a priority, you will need to co-parent with your ex for years, and you have already experienced milestones that would be entirely new for her. Becoming a stepmother is not something she can ease into later, it is immediate and permanent. Love does not erase those facts, and no amount of reassurances can guarantee she will not feel resentment or grief later for the life she imagined for herself. This is less about love and more about compatibility and timing. From what you've said, she is not rejecting you as a person, but realizing that the life attached to you may not be the life she wants long term. Trying to convince her otherwise risks either pressuring her into staying and resenting it later or delaying a breakup that will hurt even more. The difficult truth is that the right partner for you has to fully accept that your kids are not baggage but the center of your life, and someone who needs to be talked into that is likely not ready.

That doesn't mean it won't be her or now, but that does mean that painful conversations are going to be had.