In all honesty, while you and I'm sure, most people can understand her thinking of possibilities, she is already commited to a relationship, with you.
I really suggest you make her understand that, whatever either of you do with your lives, you have both made a committment and you should both be able to feel absoluteyl secure with that.
Marriages aren't based upon ifs and buts. They are based upon committment. Otherwise trust simply doesn't exist.