Even in this very shitty video angle, we can see that you're not setting your lower back in extension. Refer to the video instructions in the Technique forum. Learn to set your low back.
If you can swing it, hire a coach. I bought the book, read the book, still got it wrong.
Hired JD Shipley from Starting Strength Houston. Your not as old as I am but your not a kid either. Coaching will keep you on track with results and you'll be safer.