My guess, without all the facts, is that this mare was run on barrels previously by her old owners. She was blown up on the pattern (due to pain, or the rider, or who knows what) and then put out to pasture for 3 years so maybe she would "forget" that she blows up.
Now the OP has purchased her and is trying to run her at shows, and the blowing up problem was never fixed.
That's my guess.
If that's true, OP you may never break her of this. Some horses can be retrained, and others cannot.
Can you contact her old owners to find out the true story? Was she already blown up and that's why she was turned out to pasture? Do you think they would be honest about it (and not lie)?
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.