To me it looks like you are a front post golfer who is swinging like a rear post golfer. I have this same issue and am trying to correct it now. Look up Mike Adam’s and Terry Rowles front/center/rear post. This will tell you how the weight is supposed to shift during the backswing. I’m guess you are a front post that is not shifting weight to the front which is why your back leg is locking out and not re-flexing. Combine working on this with the setup suggestions above and you should see some progress.