I use to be precious about it, My wife just doesn't do the milage I do, when i'f met her she'd been driving for 5 years and covered maybe 30k miles, I did that in my first year. As time went on I let her drive my car when it suited me, and I got very frustrated with her driving it, the way she doesn't do certain things etc. \
But the reality is she may not drive technically better than I do, but she will certainly not take as many risks as I do so now she drives What ever car she wants as I know she won't do any harm.
Only once did she nudge a kurb, and I gave her the biggest evils I could, then checked the wheel out and there was the tinniest mark on it, I let her sweat it out for a few seconds and then let her know it's okay.
In short, I think you're just anxious, the more she drives the car the more familiar she will be and driving it un-supervised she will gain more confidence, Appreciate she may have a few near misses, but just spell it out, "You broke it you buy it" (or repair)