I know the feeling. I think your best bet would be sharing with her gently and honestly, and if that doesn't go like you hope then looking into some couples' counseling/therapy to resolve this if she's agreeable.

Family baggage is tough and there may be stuff she needs to work on within herself too before she's ready. Encouraging that is a good way to be supportive and work towards the goal of kids together.

Strongly advise against ultimatums