Explain the meltdowns, does she stop and refuse to move, buck, etc.? Horses don’t learn things instantly, like humans. They need practice. Now that doesn’t mean you should practice a flying lead change every time you cross the ring because then your horse will begin to do these things before you ask, which is not what you want. (Say you’re practicing poles on the diagonal, you don’t want your horse switching her lead before the pole because I’m sure you know that is very frowned upon in the hunter and equitation rings.) Anyway, just be patient, but don’t let her get away with it. If she gets flustered, don’t force her to do what you were working on, but try something easy. Make her trot nicely away from her meltdown (use a crop if you need to). Then try whatever you were working on once again. If she gets flustered, trot away and pat her once she’s going nicely. You want to end on a good note so you don’t end the ride frustrated (your horse will be too). When she realizes she’s doing something right, and she knows that listening to you (asking her to trot) will earn her a pat. Soon she will do as you ask. Once she’s going well at home, take her to a show. Maybe the show atmosphere makes her nervous, but that could just be a mare thing that she doesn’t like being around other horses.
"The wind of heaven is that which blows between a horse's ears."