I'm not nervous when I play, I have high sensitivity on the mouse.
Then control your mouse better. [Or lower sensitivity]
You look way too far when you go to block and attack.
It is hard to watch this video, the backpedaling and poor footwork hurts to watch, even ignoring mouse control and lack of block holding. You have a lot to work on and this issue will go away.
1. Hold your ground: You are wearing heavy armor & your weapon isn't longer, dear god why are you backpedaling? You want to give the opponent control of the fight? You want to do less damage? Also, try to not let him circle you.
2. Use your ears. Don't rely solely on eyesight as your only means of playing. Strike back when you HEAR the sound of a block. Nobody will ever hit through you again. Relying entirely on your eyes is how holds and feints get kills.