She's not stat maxed (meaning all decimal numbers over 200), and certainly not event trained. That's the main problem. Take a look at the horse that won the events you put her in:
http://www.virtualhorseranch.com/viewho ... se=1738233That is an event-trained horse. Strength, Speed, and Endurance are the needed stats for competing. as is Surefoot, and you can see that they're all WELL over 200 (the decimal, not the percentage).
To compare side by side.
Winning Horse......Yours
Strength: 203.96......94.1
Speed: 205.45......106.86
Endurance: 216.64......178.09
Surefoot: 225.562.....142.086
As a novice trainer, you will probably not be successful at event training anything other than Gymkhana (which only requires a horse to be level 25). If Endurance is the event you really want to do, look around for a sponsor trainer that offers Endurance Event Training.