Perhaps my experience can really get you down. Your fiance sounds like she has
depression, which is common in people who have migraines.
My wife has been depressed since she was pregnant with our second child 15 years ago. The birth of our third ten years ago didn't help. If it wasn't for ovulation time every 4 weeks, our sexual activity would have been pretty bleak. Even then it was gradually down hill over the years. She was nice enough to try
testosterone gel topically. It helped improve her mood and slowed the decline in libedo. Last March she decided she didn't like the 10 pounds of muscle she had gained and tapered herself off of it. (Don't stop testoserone cold turkey or things will be really bad for a while) The last time we had sex was April 6, That time and the few times before it she had a look of total revultion on her face. I decided that instead of waiting around on weekend mornings to see if she was willing to do something that she hated, my time would be better spent going out and working on our bike trails in our woods.
So we are like sister and brother, We have three healthy, athletic, smart kids, a beautiful farm, we comfortably live on my one part time job, but I am stuck with going solo with no relief in sight.
You might want to see how interested she is in joining you in some kind of counseling and get things worked out before you worry about the wedding.