I'm a recent convert of The Dog Whisperer. To paraphrase the things Cesar Millan advocates:
Often chronic fear is due to damage to the self esteem. An injury, startling situations, abuse, etc. can trigger this. Helping a dog build or rebuild self-esteem is a very, very long process.
You have to (re)build confidence slowly, and the way you build it is by helping the dog to repeatedly accomplish small goals. Practice obedience training, practice simple tricks. With every success comes a little more confidence.
He also recommends not rewarding the fear with too many cuddles. That doesn't mean you don't love them. It means you love them enough not to reinforce cringing and whimpering. Have them sit by you, and when the shaking stops - praise and reward. Distract with a task for praise like, "Sit."
Your calm state of mind is the best way to lead them to their calm state of mind. Too much cooing and worrying will elevate their anxiety. If you're worried and giving that energy, the dogs reads that it must be important to worry.
https://www.cesarsway.com/dog-care/weather-care/weathering-the-storm
https://www.cesarsway.com/dog-behavior/anxiety/persistent-anxiety-and-fear