That's a bummer, but don't let it get you down, Zoey! You are wise to do your 'homework' and ask around before buying! Been there, done that--and KNOW what a downer it can be, but have faith--you CAN find the horse that will be 'right' for you!
One thing I've come to realize is that in the world of minis, there are a number of horses advertised as being driving horses, but that the reality is that they may not really be trained to be a complete, well-experienced, and DEPENDABLE driving horse for a novice driver. EVERYBODY believes they can 'train' a mini to drive, but that simply doesn't turn out to be the case. Accepting having a cart 'attached' is NOT the same thing as being a truly 'trained' horse.
Ask questions about how the horse is to: stand quietly to be hitched, stand quietly when you enter the cart, and wait for you to ask for it to 'move off', respond to rein and whip signals quietly and correctly, understand and respond to WHOA(VERY important!), and is it quiet and not upset by traffic, dogs, noise, children, unexpected sights and sounds, and the like.Does it accept breeching?! Has it EVER 'run off', or in ANY way been 'in a wreck' with the cart? This is a very short list; in truth, I've come to believe that the PROPER training of a driving horse is more complex and more 'vital' than that of a riding horse!
You will find what you need and want; just try to be patient, and continue to do the good job of research!In the meantime-is there a knowledgable driving instructor near you, where you could learn more about the subject yourself? My recommendation would be an ADS-oriented, NOT a breed show oriented, person, so you can learn more of the genuine principles of driving, not just how to send a horse round and round a ring.
Best of luck,