I think the biggest reason you are feeling pressure is improper bar position. When I started I had a similar problem. You need to have the bar resting on the top of your rear delts. When you set up for a squat; dip under the bar, and feel the bar sit right on top of your rear delts like a little book shelf.
Keep your arms in as close as possible where it feels a little too tight. It keep the back nice and curved. Think hips and ass not quads. I you think quads forget it you will never trully enjoy a heavy squat. If you just focus on quads You would probably be better off doing hack squats. I read an article by some guru names Piloquin. He advocates having the bar up high on the shoulders. Im no expert but unless your 5' 5'' this is one bad idea. I would also stear clear of track shoes that elevate your heals. Keep those heals low.
Good Luck
Sleeper