This is what happens when you do practice works that simulate show patterns. Horses learn routines too quickly and they begin to think instead of listen.
To fix it now, you need two or three back-through obstacles at home. One should be an "L" and one should be a straight back-through. When negotiating these at home, never let a horse take more than one step at a time. Back 1 step -- stop -- go forward 2 steps -- stop -- back 1 step -- stop -- back 1 step -- stop -- go forward 1 step -- stop. Any time a horse takes a step back on his own, ride forward out of the obstacle.
When you do the 'L', be sure to stop and walk out forward before you ask for a backing step around the corner. Again, any time a horse takes a step back on his own, ride forward out of the obstacle.
I have had it take 2 hours to get a horse like this to wait on my cue to do a single straight back-through correctly. I have had it take several sessions to get a horse like this to do an 'L' correctly.
In training a trail horse, I want the 'one step and stop' rule in effect for every lateral, back-up and forward maneuver. It is much more important to have a horse listen and wait for a cue than to do an obstacle quickly in any training session.
This becomes even more important in training a reining horse. The very last thing you want one to do is 'learn' a pattern and try to do one themselves.