Googley,

I'd feel disappointed too. It sounds like she wants to be a support for you, but then the reality of her busy schedule prevents her from being as available as she needs to be. It's not wrong of you to call her, or ask her to call you back. But like you said, you need to know if she will, or will not, be able to be there for you when you need her. Otherwise, you'll end up feeling discouraged like this.

Could the two of you plan together a way that you can let her know if a return call is urgent? When she calls back, could she give you a specific time when she will try again, so that you are ready to take her call and not driving?

I think it's important to talk to her about this. Saying "call me if you need me," is nice, but you need to know that she'll be able to follow through.