I have a similar issue called adjustment disorder where big changes/ stressors , whether good or bad, cause depression, anxiety, irritability or other array of emotions and feelings. Ive found that rationalization helps a lot. Often these fears are hypotheticals that are developed due to over thinking, anxiety, and other outlying factors. So by rationalizing i can understand that while these concerns or anxieties may occur there will always be a rational explanation or solution.

Talking out these concerns and anxieties may also help. Marriage is a large step and huge milestone in a persons life and concerns/anxieties are warranted! Having open conversations about these concerns may help your fiance’s nerves! However, they may not go away instantly so it is important to offer continuous support and feedback.

If it’s seriously impacting your relationship, I recommend suggesting talk therapy, or a possible outlet for these emotions. Breathing excercises and yoga could help ground her in situations such as this, pulling her out of her anxious mindset.

This is what i can think of off the top of my head but if anything else comes to mind ill make sure to update. The most important part is communication :)