Try the following:
(1) Narrow your stance a bit. We need your heels at about shoulder width...you're a few inches outside of that. Then really force your knees out hard. This will help you be more stable, and will solve your depth issues.
(2) Take a look at your heels on every rep. You'll notice that your heel lifts up and your weight shifts inward. This is because you're trying to get depth by leaning forward. From now on, focus on sitting back, and plant your knees just about above your toes in the first 1/3 to 1/2 of the descent.
Sitting back will also help your hip drive and your excessively upright back.
Use your mid-foot as a gauge if you're doing all this right. If you feel the weight shift towards the balls of your feet, you're not doing it correctly and need to sit back more.