How do I pull my godmother out of constantly wanting to go to her childhood which no longer exists?

There are two strategies I'd recommend. One is to ask her about what is at "home," what she'll do when she gets there, who she will see, etc. Getting her talking about what she's really thinking about, which isn't the address but a feeling or specific memories, can help you understand what she needs in the moment.

The other one is always say yes. "Yes, we can head out tomorrow," "yes we'll go when the weather cools down," "yes when your dad gets done with calf season," or whatever excuse will make her feel like you're on her side and delay leaving from right now. Then distract her with a snack or activity. Once you find an excuse that works, use it over and over. You don't have to come up with a new one every time.

The problem with the reasoning that we usually want to do it that it makes them feel like we're not on their side. They think we're not trustworthy because we tell their mom isn't waiting for them or whatever thoughts they have, so it damages the relationship. If we can stay in their thoughts—in "their world"—with them, while distracting from immediate action, it helps build trust and keeps the relationship stronger.