1) Switch to front squat for a while
2) Do not go for a max, stay heavy but not maximal... do most of your work in the 80-85% range
3) DO NOT go to failure... in fact your sets should almost feel easy.. 80-85% for sets of 2-3 reps... 4-5 sets per workout.
4) Front squat 3-4x a week (NON maximal)
5) Once a week do very heavy 1/2 squats (back) starting the bar from pins with a 110 degrees knee angle. Sets of 3 reps with as much weight as you can, replacing the bar on the pins on every repetition.
GET GOOD AT SQUATTING by doing it very often without traumatizing your body. And once a week get your nervous system used to large weights.
Do this for 4-6 weeks then you will be reading to start training the squat for strength.