This isnt an uncommon problem. You know when you watch lifting video's and people are screaming "Stay Tight" a million times in the backround? You need to be doing that. Not actually screaming "Stay Tight" while you squat but, acutally staying tight while you squat. Lock in your upper back before you even unrack the bar. Imagine you just fell off a cliff and the only way you are going to survive is by holding onto the end of a rope with your scapulas while you dangle helplessly above a bunch of jagged rocks... i.e. pull your shoulder blades together as hard as you physically can... then pull harder than that.
Before you descend do two things:
- Wait for the bar to stabilize on your back
- Take a huge breath into your abdomin and hold it down as hard as you can
Dont breathe out again until you are 3/4s of the way back up or until you are completely back to starting position.
You'll know you did everything right because you will have not wobbled so much but you should feel like the internal pressure is going to melt your brain.
Stability is the reason why most people miss their squats. So, master that now and youll be well ahead of the game.