squat more, to squat more. simple, hone in on technique, when shit gets heavy muscling it out will mean nothing, when 1rm comes into play if its a true max the second u leave that "groove" you should fail, if u can fuck up your technique and still finish its not your true max and/or a weakness is stopping you from reaching it as you cant maintain good technique.
smash the GHR, strong hammies are the key.
deadlifts will build a stronger back and help you maintain positioning, also ab work will help u keep balance and aid in maintaining position.
the main thing is technique. if your training properly and still not going anywhere, u can almost guarantee your technique is holding you back.
learn the stretch shortening cycle, its very complex and timing is everything, it would be very hard to learn training by yourself, but once u got it down it makes squatting easy, to learn this i would say to go see someone who knows about it and how it works.