Well, napping in the arena or the school is her sulking or having a tantrum about what you're easing her to do.
It may be because she's uncomfortable about hacking on her own.
Or nervous about leaving your yard.
Or bored with school work.
Or uncomfortable with poor saddle fit.
Or miserable because you are causing her discomfort in her mouth.
Or a million reasons that we can't guess at without you giving us a wee bit more information.
First, check soundness, tack fit, feet, teeth and rule these out.
Then get an instructor to show you how to ride to deal with the napping. It can be done, but it takes good and consistent riding and training.
Get up, get going, seize the day. Enjoy the sunshine, the rain, cloudy days, snowstorms, and thunder. Getting on your horse is always worth the effort.