I really don't see what this has to do with you being an Asatruar. This is more of a problem for the psychology section. As I understand, she does that with everything, not just belief, right?
It would help if you stated her belief. If she's a christian, well, some denominations have this "self-sacrifice" tendency that they apply to everything. My father's like that, he will act as a martyr and allow others to walk over him.
I guess you could just place her in front of a dilemma... Example:
You: "Where do you want to go out tonight?"
Her: "I don't know, wherever you like."
"What do YOU like?"
"Um..."
"Tell me your preference. Your opinion matters to me. How about you decide once for a change what you would like to do?"
Or something. I don't know. Getting self-confidence is a process that takes years. You won't change her in a week. Just encourage her to speak her mind, to take her own decisions, and once she does, DO NOT CRITICISE them. This is very important.
She might be like that because her surroundings let her know that her opinion and her decisions are not good enough. If you support her and be understanding, she will probably learn to trust herself a bit more with time.
******
Hehe, the endless Lokeans' Tyr/Fenrir conflict? If I got a penny for every time this topic has been discussed, I'd be Soohhy rich.
Edit: JP: To each their own. As long as you're firm and steadfast in your way, it's every bit as justified as any other. I don't agree with you, but I respect your belief.
Now excuse me while I go and sharpen my sword!

:-P