You say, "I'm sorry you feel that way, but I'm not going to use money or any other valuable as proof of trust. Putting my life savings into someone else's hands is a foolish thing to do. I would never ask you to do something so huge, and I would hope you'd never consider giving away your hard-earned money to ANYONE, even me."
Also, why do you trust her to give the money back if you break up with her? I don't understand this. Why would she just give it back without any problems? If she didn't plan to use it to keep you around, then she wouldn't need it in the first place. This is some crazy irrational shit she's pulling, and you'd be the chump of the century if you gave in.