Don't worry - this is the right forum for questions like this. All of Chris's advice above is excellent.
I've watched the video. Your left-hand positioning's horrendous! (It's just like mine was before I realised that's why I couldn't move my wrists for a day after a long gig).
First off, your thumb is way too high up. It shouldn't go above half-way round the back of the neck.
Secondly, you should be using your thumb to pivot when you go from string to string (and from low to high on the fretboard).
Thirdly, get your elbows out. They're wedged into your ribs at the moment. Move them out away from your body.
Fourthly, try to keep your wrist as straight as possible (in line with your forearm).
And then last of all, use your ring finger together with your little finger to fret the octave. It might help, it might not.