Do your squats in rep range of 8-12 with good form, until your form breaks down. = this means lower weight, can you handle that?
Then, jump on the Hack-Squat and perhaps go in the 5-8 rep range for 3-4 sets, really push your ass into the seat for every rep.
- put your feet as "high" up as you can.. Half my foot is always un-supported/off the mat.
Then do stomach vacuums outside of the gym.
That's what I'm doing to solve something similar. And by similar I mean, my form breaks down on the last set(front squats) which is 3-4 reps.
I could be wrong but that's what I'm doing for it.