I know you don't want to hear it but the best exercise to learn to set your back is the deadlift. I saw on your other posts that you went and seen a coach and you reported that you were sorted out then. My guess is the weight got heavy again and you are losing your back again. It sucks but you may have to reduce a bit again to hammer the form then work your way back up hopefully passing you old spot. Remember your lower back and your hamstrings are in a battle that the lower back must win. Best of luck to you.