Ok.....looks like your bracing nice and hard with your legs.........here's some tips,
Curl your mid section, your leaning too far back, so sit on your pockets, brace and do a tummy curl a little more so your not leaning straight back. Also bring your hands further out in front of the saddle horn, ideally you want no contact on his face at all during a stop, however there are some horses that need the contact.
So, heels down, sit on pockets, abdominal crunch (curl forward a bit) hands soft and forward........to get him sucking under during the stop, ask for the stop then immediately pick up your reins and back him up so he's thinking back up when you say 'whoa'....looks like you're getting there though....
Hope that helped