1. Give it time! Don't make a snap decision.
2. What is the likelihood that your sorority will go through with the nationalization? Do the current nationals support you in this? Or is it just partly your own members that are balking at the idea? Promote nationalization to the younger members. Explain why you want what you've worked so hard for!
3. What would be the time frame for nationalization? If you want to participate in a national group before you graduate, you have to keep that in mind.
This a tough decision...
Best of luck!