Have you considered taking the train (technically commuter rail) to Newburyport from Boston and then renting a car from there? It's quite close to where you're going and would probably allow you to avoid dealing with navigating the city in a car alone at all. You would just have to cab to north station instead, and could do the reverse when you come back. I also get some driving anxiety especially in the city (though less severe), and I totally understand wanting to minimize putting yourself in a more stressful situation than necessary.