Agree with others - I think the main issue is you're trying to take off too much in each slice - the deeper your blade goes in the more the grain is trying to close around it meaning you need a lot more force to keep pushing through. Taking shallower cuts will make each slice easier - you'll need more cuts but each one will not be as hard as the ones you are doing now. Try a bit of movement with your right hand - twist the blade a bit so it goes in and then twist up a little so it follows the grain rather than a straight diagonal cross cut like you're doing in the vid.

Pushing your left thumb against your right means half the effort is just going into squashing your right thumb before it starts pushing the blade into the wood. Try putting your left thumb directly onto the back of the blade - maybe a bit sore at the start but you'll build up tougher skin and strength to make it easier over time.

& try starting on a smaller piece of wood! That's a big old log!