I have been through it. It was the hardest time in my life. I picked up a few habits that I have since beaten but that demon always haunts me.

If you lover her and she loves you then keep letting her know that even with every thing she has going on in her life right now your still there for her and will always be. Remember your were about to say "For better or worse" why not show that you mean thats what you want regardless of whether you say the words if its what you want then prove it. Be there for her be her sounding board while still being her rock, a man should be the high place in the flood and a dry place from the storm when times like this happen. This is your test if what you two feel is real work through it getting married is only a drop in the bucket of what happens between to loving people so don't hold that against her. 99.995 percent of women aren't hard to figure out most of the time it us guys not doing the things we think are unmanly that makes us think they are such a mystery.

Jealousy is a child's game and has no place in an adult relationship serious enough for marriage. Again let her know that if she wants to call off the wedding so be it but this thing that she is going through is something that the TWO of you need to get through together.

Hope that helps.
Best of luck