If she is anything like my mare she will be naughty in a prelim test as there is just too much time for her to think! A good option if you want experience is to ride HC in a higher grade class. That means that you pay entry fee and get judged but you are not competing for places or rosettes. It is quite a good way of taking pressure off you as a rider while acquiring competition arena skills, lets the judge know you and/or horse are inexperienced so they should give you better feedback, most places will not even post your score (so no embarrassment factor if you make a **** up) and will let you have your sheet before the end of the class.
Definitely worth having a go at a Novice, I usually compete one - two levels lower than the horse is working at home. My 'baby' is currently competing Novice and Elementary and working medium/advanced medium at home - her canter half-pass and changes are not good enough for competition yet, more work needed