1) stop staring at the mirror. Stop. Staring. At. The mirror.
Your hea position should be neutral. Find a spot on the wall that's about 6 feet away and fix your sight there.
2) you have a butt-wink. In other words, your lower back is rounding at the bottom, which causes the wink as you enter into lumbar extension as you back up. You have to consciously think of sitting back instead of squatting down, which will force your hamstrings to extend at the bottom so your lower back remains fixed. In your current situation, your hamstrings shorten at the bottom pulling your hips inwards, causing the butt-wink