A Schumacher video won’t provide a whole lot of help. It’s so old that the chassis designs have changed, and the driving style is too advanced for a beginning driver.
As far as line goes, if it feels like a perfect arc, you’re probably apexing too early. In general, the apex of your turn should be later, so that you can get on the gas sooner and utilize more of the straight after the turn.
For the grip issue, along with changing your driving, it seems like the kart may be binding up and keeping the inside rear wheel on the ground. If this is the case, you may want to narrow the front track width.
As far as driving goes, counter-steering is always bad. Any time you turn in the opposite direction you scrub speed from the kart, slowing down. You want to focus on being smooth with your steering inputs, and only having to make two movements through a turn: turning in and straightening out. Driving on the limit, it’s not easy, but the more seat time you put in the more comfortable it will feel and naturally it will come.