There are tons of youtube videos that can show you how to get the pattern on your mare. But if there are holes in the fundamental training then those should be looked at first. It sounds like the mare has been allowed to head back to the gate and has learned that if she heads back to the gate then the event is over. I've never had this issue, but I have heard the advice given that working them harder by the gate can sometimes get that idea out of their minds.
And yes there are tons of exercises that aren't the pattern that can help out. I don't know if I am allowed to put the link, but there is a site called Barrel Racing Tips. She has tons of ideas an things that you can try for a plethora of problems.
to ride on a horse, is to fly without wings