I've never roasted one before, so operator error is highly likely. I followed a recipe I found online that said just place it on a rack in the roasting pan in an oven set to 450, and as soon as you put it in the oven, reduce the temp to 350 and roast for an hour or until internal temp reaches 160. I carefully monitored it with a cooking thermometer so I know I didn't cook it for too long. The recipe said don't bother basting it since it's only in the oven for an hour or so. What did I do wrong?
That sounds like the roasting method for a whole bird. Starting that high and going for an hour sounds like it would be guaranteed to dry out just a breast, especially if it's not on the bone.
Doing just the breast by itself is going to be hard. I'd look for a different recipe, and I agree with PPs that brining it will be the way to go.
Or try a non-roasting method, like braising or slow cooking.