You do not set your back consistently in extension.
Your knees are very bent. If you keep the bar one inch from your shins in a standing position, you will be able to bend your knees less, your hips will be higher and you will be able to extend your spine.
That weight is no challenge for you. You lift it easily and do not need good form to do so. As you add weight, you will discover the need to practice good form at light weights.