Vigorous, sweaty exercise can help burn off some of the excess adrenaline caused by anxiety and stress. So when you feel nervous, jittery, anxious or stressed-out before boarding a flight to Monrovia, Liberia, go for a jog or go to the gym and work out, or go swimming, or do some vigorous dancing or some other form of aerobic activity for at least one hour so that you are all sweaty and tired, and that will help relax you and calm your nerves.

And stop using stimulants of any kind: caffeine and nicotine are stimulants. No more coffee, no more cigarettes, no more caffeinated teas, "energy" drinks, caffeinated soft drinks, chocolate, and even some headache medicines have caffeine added.

That's my recommendation; I bet that increased exercise time and avoiding stimulants will help you.