Benzodiazepines (Diazepam) are not advised for use to overcome fear of flying
some reasons for this include:
Sedation and therefore impaired ability to follow instructions in event of an emergency.
Increased sedation and lack of movement and therefore increased risk of DVT
Occasionally diazepam can cause agitation rather than sedation.
Benzodiazepines (Diazepam) are banned in a some countries.
There are a number of useful courses to help you tackle this phobia
Fear of flying course | Fearless Flyer (easyjet.com)
Flying Without Fear | Courses to Conquer Your Fear of Flying
Fear of flying courses from British Airways™ | Flying With Confidence