You want confidence? Go practice what makes you nervous. One of the worst things for new tandem teams is stopping and starting. So, whenever Silly and I take a new team out, we find a parking lot and make them practice.
If it's traffic trouble, find some neighborhood/park streets that have some traffic and ride. As you do it more, seek out roads with a bit more traffic. Having a
helmet mounted mirror does wonders for some folks. Lots of folks I know that hate traffic felt MUCH better when they could see what's going on behind them. Helmet mounted mirrors are IMO the best way too. You can look back without changing anything, and they don't flop around like the ones that mount on your glasses.