What Sheepfarmer said...
What I did with my wife was put her in low range and 1/4 throttle until she got comfortable with the HST machine. Even if she goofed up three times in a row, she wouldn't go anywhere.
And this is a woman who did well learning a clutch on my '83 Z28 but refused to drive it in traffic. Her confidence never matched her ability. Somehow, she learned how to ride a motorcycle (I got her started and then sent her and my daughter to MSF school for the weekend.) But then she whined for a Vespa because they have no clutch. ("Yes Dear!")
Confidence comes with experience. She made a boo-boo and will pay more attention from now on. Don't ban her from it,
make her run the machine every day you have off for the next month! She'll be a lot safer and you won't worry about her near as much.
Motorcycling and flying were two experiences that matured my daughter. Basically, the teaching program for both is: "Do this and you are safe. Do this and you will get hurt." Works for tractors too.
Have fun and enjoy that new FEL... I can't think of a nicer wedding anniversary present.